Role of maleic- anhydride-grafted- polypropylene in supercritical CO 2 foaming of poly (lactic acid) and its effect on cellular morphology

Abstract

Poly (lactic acid)/maleic-anhydride-grafted-polypropylene (PLA/MAPP) blends were prepared by melt blending method. The effect of MAPP content on the dispersion morphology, thermal properties, and rheological behavior of PLA/MAPP blends was studied. Then PLA/MAPP blends were foamed using supercritical CO₂ as physical blowing agent; and the cellular structure, cell size, as well as cell density were investigated. The results showed that of MA reacted with PLA, and thus a small amount of branched polymer would be formed. The branching structure strongly affected the rheological behavior, as well as the thermal properties of PLA. The blending morphology of PLA/MAPP blends also had a significant effect on the cell density of all the samples. The results indicated that homogeneous and finer cellular morphology for PLA/MAPP foams with high expansion ratio could be achieved with a proper content of MAPP in the blends.

Keywords

Poly (lactic acid)maleic-anhydride-grafted-polypropylene foam blend

Introduction

Poly (lactic acid) (PLA) is a kind of biodegradable polymers, which is synthesized through the ring opening polymerization of lactide.^1,2 Recently, PLA has been used as an important bio-based polymer in the biomedical and environmental products since its merit of bio-degradability, bio-compatibility, and non-toxicity.^3,4 Among these PLA products, PLA foam is very attractive due to its heat insulation, appealing physical-and-mechanical properties, low density, and relatively low cost. Through the foaming technology, the ductility and impact resistance of PLA could be efficiently enhanced by providing a significant expansion ratio and weight reduction.^5,6

However, lack of adequate foam-ability is one of the most serious defects that restricted the feasibility of PLA foaming process. The poor foam-ability or melt strength was associated with the low molecular weight and linear molecular structure of PLA. In order to improve the melt strength, thus enhance the foam-ability of PLA, some modification methods have been studied, for example, increasing the molecular weight, inducing the branched molecular structure, and blending modification, etc.^7–10

Among these methods, the molecular modification (increasing the molecular weight, and inducing the branching molecular structure) was considered to be the most efficient way to improve the melt strength of PLA. The molecular structure, especially the branched molecular structure, predominantly affected the rheological properties, such as melt elasticity and melt strength. The longer terminal relaxation time, and thus the better rheological property, could be obtained with the higher molecular weight and proper branching degree. Wang et al. synthesized the long chain branched PLA (LCB-PLA) by using the hexamethylene diisocyanate (HDI). The effect of the molecular structure of LCB-PLAs on the rheological behavior was investigated. They found that the import of LCB structure improved the rheological properties, such as storage modulus, and complex viscosity.¹¹ Di et al. prepared LCB-PLA in the melt state. In their work, a small amount of diol (1, 4-butanediol) was firstly reacted with the linear PLA chain, and then diisocyanate (1, 4-butane diisocyanate) was used to link the chains together through the chemical bonds. The chain extended PLA (CEPLA) showed an enhancement on melt elasticity.¹² We have also prepared branched PLA using multi-epoxy group compound as chain extender, and studied the effect of branched molecular structure on its foaming behavior. The fine and excellent cellular structure was obtained with the branched molecular structure.¹³

Besides that, blending with other polymers was also considered as a feasible method to overcome the poor foam-ability of PLA for developing a lot of novel polymer products. And it could balance the properties of various polymeric components to eliminate the weakness on some properties of PLA. Wang et al. prepared PLA/poly (ɛ-caprolactone) (PCL) blends through melt blending method using triphenyl phosphite as a reactive compatibilizer. They found that triphenyl phosphite was an effective coupling agent in the reactive compatibilization of PLA and PCL, and the elongation at break of blends was obviously improved.¹⁴ Previously, we had fabricated PLA/poly (ethylene terephthalateco-1, 4-cylclohexylenedimethylene terephthalate) blends by using epoxy compound as a reactive compatibilizer. The results revealed that the dispersion phase morphology could affect the cellular structure of PLA foams, resulting in a higher cellular density and smaller cell size.¹⁵

In this paper, we prepared a series of PLA/maleic-anhydride-grafted-polypropylene (PLA/MAPP) blends for foaming purpose. Firstly, the second polymeric chains, MAPP, could react with PLA resulting in a branched molecular structure, which would increase the rheological properties of PLA. MAPP have small amount of maleic anhydride (MA) groups (1.0 wt%) on the PP backbone. The MA groups could react with the end hydroxyl groups on PLA chains. Consequently, a small amount of branched structure with a backbone of PP and the branching chains of PLA was formed in PLA/PP blends. The branched structure of polymer made PLA has noticeable foam-ability and remarkable cellular structure. Secondly, a dispersion/matrix-phase-morphology was formed while blending PLA with MAPP. The blending morphology would also affect the cellular structure. The aim of this work is to investigate the effect of PLA/MAPP composition on the foaming property and cellular structure of its foam. The dynamic shear rheological and crystallization properties were characterized to study these effects.

Materials and methods

Materials

The biodegradable linear PLA (2002D) with a MFR of 5.6 g/10 min was purchased from NatureWorks. It is a semi-crystalline polymer containing approximately 4% d-lactic. The MAPP (containing 1.0 wt% maleic anhydride), under the commercial names of Polybond 3200, was supplied by Chemtura, Inc., USA.

Preparation of PLA/MAPP blends

The PLA/MAPP blends were prepared in a Haake internal mixer at 190℃, with a mixing time of 10 min and mixing speed of 50 r/min. Prior to melt mixing, PLA and MAPP were pre-dried in an oven at 60℃ for 4 h to remove excess moisture. Afterwards, the blending samples were compression molded into sheets of 2 mm in thickness for subsequent characterizations and solid-state batch foaming. The PLA/MAPP blends with 0%, 1%, 3%, 5%, and 7% MAPP were prepared.

Supercritical CO₂ batch foaming

The PLA/MAPP blending foams were prepared by the batch foaming method using supercritical CO₂ as physical blowing agent. In order to investigate the effect of MAPP content on the foaming behaviors and dispersion phase morphologies of various PLA/MAPP blends, all the samples were prepared and foamed under the same conditions. First, the samples were put into autoclave at a temperature of 170℃ and a high pressure of 20 MPa for 4 h. After the CO₂ fully was diffused and dissolved in the polymer melt matrix, the samples were cooled to the foaming temperature of 130℃. An instantaneous pressure drop by release of CO₂ from 20 MPa to 0.1 MPa provided the driving force for cellular nucleation and growth. At last, the foaming samples were prepared for further characterizations.

Characterizations

Differential scanning calorimetry (DSC)

The cold crystallization and melting behaviors of various PLA/MAPP blends were studied by DSC (Q100, TA, USA) system purged with nitrogen. The samples were heated to 200℃ at a temperature rate of 10℃/min.

Polarizing optical microscope (POM)

Spherulitic morphology of the samples was observed by a polarized microscope (POM, BX-51, Olympus, Japan). The samples were heated at the temperature rate of 30℃/min from room temperature to 200℃ and left for 5 min, and then cooled down at the temperature rate of 10℃/min to 130℃ and left for 30 min. After that, samples were further cooled down to 40℃ to observe the changes of spherulite morphology. The magnification ratio was chosen as 400.

X-ray diffraction (XRD)

Investigation on the crystal form of PLA, MAPP and PLA/MAPP blends was carried out on a XRD instrument (D8, Advance-TXS, Bruke AXS) using a Cu Kα radiation at a wavelength of 0.1541 nm and operating at 40 kV and 40 mA. Scans were performed with a step size of 0.06° and a scan speed of 0.1 s/step from 2θ = 10–50°.

Fourier transformation infrared spectroscopy (FTIR)

Infrared spectra of PLA/MAPP blends were obtained using an FTIR (Nicolet iS10 thermo scientific spectrometer) in transmission mode. Each spectrum was obtained within the range of 3800–480 cm⁻¹ with a wavelength resolution of 4 cm⁻¹.

Shear rheometry

Shear rheological behaviors of various samples were tested using a strain-controlled rheometer (ARES Rheometer, TA, USA) at 190℃, with a parallel plates (20 mm in diameter with a gap of 1.0 mm). The frequency range was 0.1–100 rad/s, and the maximum strain was fixed at 5%, in order to confirm that these conditions were within the linear viscoelastic region under nitrogen. The complex viscosity (η*), storage modulus (G′), and loss modulus (G″) were measured at various frequencies.

Foaming properties

The dispersion morphology of PLA/MAPP blends and cellular morphology were investigated by a scanning electron microscope (SEM, FEI Quanta FEG) at an acceleration voltage of 5 kV. The cryo-fractured sample surface was coated with Au-Pd before microscopy observations. The magnification was 3000 and 200 for observing dispersion morphology and cellular morphology, respectively. The densities of both prefoamed, ρ, and postfoamed, ρ_f, samples in g/cm³ were measured by a pycnometer (Quanta-chrome Instruments, USA).

Results and discussion

Reaction between PLA and MAPP

Figure 1 shows the FTIR spectra of MAPP and PLA/MAPP blends. In Figure 1a, the peak at 1717 cm⁻¹ is related to the dimeric form of dicarboxylic acid; the relatively small peaks at 1786 cm⁻¹ and 1862 cm⁻¹ are characteristic of the symmetric vibration and anti-symmetric vibration of C=O in cyclic anhydride.¹⁶ Figure 1b illustrates the effect of MAPP content on FTIR-transmission spectra of PLA/MAPP blends. In these FTIR spectra, the lower intensity peaks in the range of 2100–2300 cm⁻¹ and the intensive peak around 1700 cm⁻¹ represents O–H stretching vibrations in carboxyl acids groups and C=O stretching vibrations of PLA, respectively. Double shoulder peaks from 1250 to 1050 cm⁻¹ and peaks at 1300–1500 cm⁻¹ are related to the vibration of C–O from carboxyl groups, the stretching vibration of C–O–C, and the vibration of C–H in –CH₃. The broad peak around 3000 cm⁻¹ is associated with the stretching vibrations of O-H groups, and the peak at 1630 cm⁻¹ is related to the bending vibrations of H–O–H.¹⁷ In polymeric blending study, MAPP was usually used as a compatibilizer to improve the blending morphology. MAPP could promote the dispersion of blending polymer (such as poly (trimethylene terephthalate)/polypropylene,¹⁸ polyamide/polypropylene¹⁹) because MA groups could react with hydroxyl group. Kim et al. used MAPP to enhance the interfacial adhesion of PP and bio-flour, which is firmed by FTIR and SEM.²⁰ Theoretically, the MA groups on MAPP could react with end hydroxyl groups of PLA. The branched PLA-MAPP would be formed through the reaction as shown in Figure 2. The two peaks around 1786 cm⁻¹ and 1862 cm⁻¹, which is related to the vibration of cyclic anhydride group, cannot be found in the spectrum of the PLA/MAPP blends, as shown in Figure 1c, indicating that the MA groups reacted with PLA, and had been consumed.

Thermal behavior of PLA/MAPP blends

While the branched polymer was formed through the reaction between MA groups and hydroxyl groups, the thermal behavior of PLA, which has profound effect on the foaming process,²¹ was affected. Generally, crystallization parameters play the critical role in controlling the degree of expansion ratio and the integrity of cellular structure. Especially under compressed CO₂, the blowing agent would significantly increase PLA’s crystallization rate.²² The foaming temperature is usually close to the crystallization temperature. Thus, the changes in crystallization behavior will directly affect the viscosity of PLA during foaming process; consequently affect the foam-ability of PLA.²³

Figure 3 shows the glass transition, cold crystallization, and melting DSC curves of pure PLA and PLA/MAPP blends. The corresponding parameters, glass transition temperature (T_g), cold crystallization temperature (T_cc), melting temperature (T_m), and the specific enthalpy (ΔH_cc and ΔH_m) are given in Table 1. The crystallinity of PLA could be calculated by equation (1)^24,25:

X_{c} = Δ H_{m} / Δ H_{m}^{0} \times 100 %

(1)

where ΔH_m is the enthalpy for melting,

Δ H_{m}^{0}

is the enthalpy of melting for a 100% crystalline PLA sample, which is 93.7 J/g.²⁶

Table 1.

The crystallization parameters of PLA and PLA/MAPP samples.

PLA/MAPP Blend ratio	100/0	100/1	100/3	100/5	100/7
T_g (℃)	61.75	60.02	61.89	60.03	60.68
T_cc (℃)	125.98	112.66	112.26	108.63	107.22
H_cc (J·g⁻¹)	16.49	23.12	21.67	19.72	19.69
T_m (℃)	152.11	154.21	155.97	155.20	155.28
	–	150.15	151.31	149.92	149.15
H_m (J·g⁻¹)	21.22	23.86	24.31	24.51	26.15
X_c (%)	5.05	0.79	2.82	5.11	6.89

MAPP: maleic-anhydride-grafted-polypropylene; PLA: poly (lactic acid).

The enthalpy of cold crystallization should be subtracted from the enthalpy of melting, in order to analyze the crystallinity during the cooling process. Thus, the modified equation could be written as following:

X_{c} = (Δ H_{m} - Δ H_{cc}) / Δ H_{m}^{0} \times 100 %

(2)

where ΔH_cc is the cold-crystallization enthalpy.

It could be seen from Figure 3a that the T_g of PLA samples, which were similar to that of pure PLA sample at about 60℃, are not obviously affected by adding MAPP. The T_cc of pure PLA is at 125℃, showed in Figure 3b. For the case of PLA/MAPP blends, the T_cc shifts to the low temperature, from 125℃ to 107℃, with the increasing content of MAPP. Cold crystallization enthalpy of PLA/MAPP blends shows an increment at the MAPP content of 1%, and then decreases with increasing MAPP content. This phenomenon might be attributed to the formation of branched polymer. The existence of branching point would decrease the activation energy of crystal nucleation, and thus the cold crystallization would occur at relatively low temperature.

DSC curve of pure PLA possesses a melting peak at a temperature of about 152℃, as shown in Figure 3c. For PLA/MAPP blends, DSC curves exhibit two melting peaks, with a higher temperature melting peak at around 154–155℃ and a lower temperature peak at around 149–150℃. Besides that, with the increasing MAPP content, the melting peak at higher temperature gradually become major melting peak, while the melting peak at lower temperature tends to be a shoulder. Generally, it is common to observe a double melting peak in polymers, which might be attributed to several reasons reported in some research papers.^27,24 However, in this study, the double melting peak behavior observed might be attributed to the different crystalline morphology. In order to study the double melting peaks, the XRD was used to characterize the crystal form of the two melt peaks as shown in Figure 5. The results showed that the crystals only related to α crystal form of PLA. Some authors suggested that the bimodal melting peaks of PLA might be attributed to the melt-recrystallization phenomenon.²⁸ In the heating process, the small and imperfect crystals formed successively through the melt–recrystallization mechanism. The imperfect crystals did not have enough time to reorganize into crystals with higher structural perfection; thus the part of imperfect crystals would remelt at relatively lower temperature.

Besides the crystallization temperature, the degree of crystallinity of PLA/MAPP blends also has a profound influence on controlling the cellular morphology of their foams. A faster crystallization rate and higher degree of crystallinity would help in solidifying the cells in a short time during the foaming processes. Wang et al. found high-expansion-ratio microcellular PLA foams could be stably produced by controlling crystallinity.²⁹ As summarized in Table 1, the addition of MAPP in the PLA would affect the degree of crystallinity of PLA. While blending PLA with small amount of MAPP, the overall degree of crystallinity decreased from 5.05% to 0.79%. With the increasing content of MAPP, the degree of crystallinity of PLA gradually increased and reached 6.89%.

POM was used to study the relationship between MAPP contents and morphology of spherulites. Figure 4 shows the POM micrographs of pure PLA and PLA/MAPP blends isothermally crystallized at 130℃ for 30 min. Compared with pure PLA, the spherulites number for the series of PLA/MAPP blends increases tardily with the increasing amount of MAPP, while the spherulites size remains steadily. This phenomenon is attributed to the nucleating effect of branched chains, which provide more heterogeneous nucleating points.³⁰ As a consequence, nucleation density of PLA could be slightly increased by adding MAPP.

Figure 5 showed XRD curves of pure PLA, MAPP, and PLA/MAPP blends. In Figure 5a, the pure PLA had a strong reflection at 2θ = 17° due to diffraction from (200) and/or (110) planes, and the other reflection around 2θ = 19.5° is related to (203) plane. These reflection peaks indicates that the pure PLA sample had the typical α-phase crystal form. For pure MAPP, the peaks appeared at 14.1°, 16.9°, 18.5°, 21.2°, and 22° are associated with (110), (040), (130), (111), and (130) planes, respectively. These refection peaks demonstrates the formation of α-phase crystal of PP. In order to further characterize the crystal form in POM micrographs, the XRD spectra of PLA/MAPP blends with various MAPP contents were tested. As shown in Figure 5b, all the four XRD curves of PLA/MAPP blends shows a strong reflection peak around 2θ = 17° and a relatively small reflection peak around 2θ = 19.5°, which is related to the α-phase crystal form of PLA. The curves indicate that the spherulites of PLA/MAPP blends possess PLA α-phase crystal form.

Rheological behavior of PLA/MAPP blends

Dynamic shear rheological testing is a direct way to characterize the rheological properties of PLA/MAPP blends. Shear rheological behavior is relative and sensitive to the molecular chain length, topological structure, and blending ratio. To analyze the rheological behaviors of PLA/MAPP blends, three rheological parameters, namely the complex melt viscosity (η*), storage modulus (G’), and loss factor (tan δ), versus various frequency were studied.

Figure 6a illustrates the effect of MAPP content on storage modulus G’ of PLA/MAPP blends at differential angle frequency. All G’ − ω curves have a similar trend that storage modulus increases with increasing frequency. The improvement of melt elasticity could be reflected by the increase in storage modulus, especially in low frequency zone. The higher the G’ value is, the better the melt elasticity is, and thus the better the foam-ability is. When adding MAPP to PLA matrix, the hydroxyl groups on PLA chain ends could react and link with the 1.0 wt% MA groups on MAPP, and the segment length and topological structure of PLA would be changed, leading to the change on viscosity and the elasticity of the PLA melt.

From Figure 6a, it is found that the position of all the G’ − ω curves of PLA/MAPP blends with different MAPP contents is higher than that of pure PLA, which implies that the melt elasticity of PLA/MAPP blends is obviously higher than that of pure PLA. The PLA/MAPP blends with high MAPP content have a longer terminal relaxation time. This trend is induced by the formation of small amount of branched polymer. Some researchers also found the similar shear rheological behaviors in the branching polymer, such as PP.³¹ The melt elasticity, which is reflected by G’, is improved by adding MAPP. This is attributed to more entanglements, and longer chain branched structure. Generally, the foam-ability or melt elasticity is strongly related to the storage modulus value. The improved G’ of PLA/MAPP blends could provide better foam-ability. Moreover, at high frequency region, G’ tends to be similar due to chain orientation under intense shear field. This is also an evidence of the branching efficiency.

Figure 6b illustrates the relationship of the complex melt viscosity η* of PLA/MAPP blending samples as a function of angular frequency. For all the blending samples, the η* values gradually decrease with angular frequency due to shear-thinning behavior, as shown in Figure 6b. The η* of PLA/MAPP samples increase drastically with the increasing MAPP content at low angular frequency, which manifests the great effect of MAPP on the η* of PLA. Compared with that of pure PLA, the complex viscosity curves of PLA/MAPP blends have a steep slope and there is no obvious Newtonian plateau at low frequency, which could be attributed to the reaction between PLA and MAPP, and the formation of branched structure. The higher complex melt viscosity could prevent cell rupture better during cell growing stage.

Loss angle δ refers to the angle that the strain lags behind the stress while the melt is exposed to an alternating tress field. The loss factor tan δ is defined as the ratio of G’’/G’, and is also known as a ratio of viscous to elastic contribution at a given angular frequency. The smaller the loss factor tan δ is, the faster the melt responds elastically, and thus the higher the melt elasticity is. Figure 6c depicts the tan δ of PLA/MAPP blends at various angular frequencies. For all PLA/MAPP blends, the trends of tan δ remain similar. The tan δ decreases with frequency. While the tan δ curves of PLA/MAPP blends are much lower than that of pure PLA under same frequency, indicating that the melt elasticity is improved by adding MAPP. This could be attributed to the fact that the increase of branched structure enhances the number of entanglements between chains. These entanglements could be acted as physical network points, enhancing the melt elasticity of PLA/MAPP blends.

Morphology of PLA/MAPP blends and foams

The dispersion morphology of the cryo-fracture surface of PLA/MAPP blends was investigated by SEM, as shown in Figure 7. These SEM micrographs show the dispersion morphology of PP in PLA matrix. The spherical dispersion phase had been separated during the melt mixing and uniformly dispersed in the PLA matrix. As displayed in Figure 7, the size of dispersion phase increases remarkably with the increasing content of MAPP in PLA/MAPP blends. The dispersed phase size of PLA/MAPP blend contained 1 wt% MAPP is smaller than 1 µm, and that of blend contained 3 wt% and 5 wt% MAPP is about 2 µm and 4 µm, respectively. Besides, the size would increase to over 6 µm when adding 7 wt% MAPP into PLA.

Generally, the dispersion morphology of PLA/MAPP blends is controlled by two factors: the interfacial adhesion and the blending ratio. A branched polymer could be formed, attributed to the reaction between MA groups and hydroxyl groups on PLA chains. The branched polymer could reduce the interfacial tension due to its good compatibility with both PLA and PP. Each block segment of PLA-PP polymer would selectively entangle with its blending phase in the interfacial region. These entanglements would efficiently improve the dispersion of PP. As a consequence, the compatibility of PLA/MAPP would be improved. However, the branched polymer present in the blends is too low. Besides that, blending ratio of PLA/MAPP blends also has a significant effect on the dispersion morphology. While MAPP content reaches a certain level, PLA/MAPP blends would form a typical dispersed/matrix phase morphology due to their immiscibility. For the PLA/MAPP blends, the dispersion morphology is largely controlled by the blending ratio. The over dose MAPP dispersed phase approaches each other more easily during the mixing process in the PLA matrix, and thus the dispersion phases would collide and coalesce. Therefore, the MAPP dispersed phase size increases sharply.

The PLA/MAPP blends were foamed in an autoclave using supercritical CO₂ as blowing agent. In the autoclave, CO₂ would dissolve into PLA/MAPP blends under high temperature and pressure; hereafter, the CO₂ solubility reduced by thermodynamic instabilities, usually a pressure drop, to induce the cellular nucleation. After nucleation, the bubble growth started, which was mainly controlled by the temperature and pressure setting. The cellular morphology was finally vitrified when the temperature was reduced to room temperature and the pressure decreased to ordinary pressure. Cellular density was analyzed by using software image tool and calculated.³²

The cellular morphology and foaming parameters of PLA/MAPP blends are showed in Figure 8 and Table 2, respectively. As shown in Figure 8, PLA/MAPP foams present the orientation. The phenomena are associated with the permeation difference between the air and CO₂ in foams. After the formation of PLA/MAPP foam, the CO₂ began to exchange with air in the autoclave under the setting foaming temperature (130℃). During this time, pressure losses occurred in the cell, which can, in turn, generate foam thickness losses and result vertical direction due to the gravity force. As shown in Figure 8, the pure PLA foam exhibits a poor cellular structure, which characterized with non-uniform and irregular cells. Its cells remain a spherical shape, and some cellular membranes become ruptured. Moreover, the expansion ratio and cellular density of pure PLA is only 3.90 and 7.96 × 10⁶ cells/cm³, respectively. The poor cellular structure and low expansion ratio of pure PLA are associated with its poor foam-ability and low melt elasticity; and the low cellular density could be ascribed to the homogeneous nucleation of pure PLA. While adding MAPP into PLA, it could be observed that the cellular morphology of PLA/MAPP blends has been drastically improved. With the increasing of MAPP content, the expansion ratio of PLA/MAPP blending foam increases obviously. According to the rheological studies of PLA/MAPP blends, the melt elasticity of PLA is gradually improved by the increasing MAPP content; thus, the foam-ability of PLA is enhanced; and finally, the expansion ratio of PLA/MAPP is increased. For the PLA/MAPP blend with 7 wt% MAPP, the expansion ratio could reach 23.52, and a typical dodecahedron cellular morphology can be obtained.

Table 2.

The cellular parameters of PLA/MAPP blending foams.

PLA/MAPP Blend ratio	Density (g/cm³)	Expansion ratio	Cell size (µm)	Cell density (cell/cm³)
100/0	0.318	3.90	156.25	7.46 × 10⁶
100/1	0.130	9.53	143.83	5.33 × 10⁷
100/3	0.075	16.42	128.59	8.63 × 10⁷
100/5	0.063	19.64	119.66	2.36 × 10⁸
100/7	0.053	23.52	89.56	7.35 × 10⁸

MAPP: maleic-anhydride-grafted-polypropylene; PLA: poly (lactic acid).

Table 2 presents the average cell size and cell density of PLA/MAPP blends with the increasing MAPP content. Pure PLA with poor melt elasticity has the largest average cell size of 156.25 µm. When PLA was blended with MAPP, the cell size would decrease to 89.56 µm with the increasing MAPP content. Cell density also had a similar trend with cell size. As shown in Table 2, the cell size increases with increasing MAPP content, and reaches a maximum value of 7.35 × 10⁸ cells/cm³ for PLA/MAPP (100/7) blend. These trends of cell size and cell density could be ascribed to the change of cellular nucleation mechanism and the dispersion morphology of PLA/MAPP blends.

In the classic cellular nucleation theory, there are two types of nucleation way: homogeneous nucleation and heterogeneous nucleation. Homogeneous nucleation means the formation of gas phase in the polymeric matrix phase, which is occurred when a sufficient number of dissolved gas form clusters for a long enough time to make a critical cell radius. In the other way, the heterogeneous nucleation represents the critical cell formed on the surface of some additives (inorganic fillers or second polymer phase) in polymeric matrix. However, the activation energy of heterogeneous nucleation is much lower than that of homogeneous nucleation. Usually, people use inorganic fillers, such as clay, silica, or carbon nanotube, as cell nucleation agent, and the cell nuclei would form on the surface of inorganic filler. Recently, Hartland et al. researched the thermodynamics model of bubble nucleation on the interface of two immiscible liquids.³³ They found the bubble nucleation was easier in the interfacial area. And the nucleation effect was greatly depended on the surface tension of each liquid phase.

It was found that the cellular nucleation in the PLA/MAPP blends was heterogeneous nucleation. Cells nucleated more efficiently in the interface area. As shown in Table 2, the cell density values of all the four PLA/MAPP blends are higher than that of pure PLA. The value of cell density shifts from 10⁶ to 10⁸ with the introduction of interface phase, indicating the excellent heterogeneous cell nucleation effect. As a consequence, the finely controlled and improved cell morphology of PLA blending foams could be obtained.

Conclusions

In this paper, PLA/MAPP blends were prepared by melt extrusion. The FTIR results of PLA/MAPP indicate the formation of PLA-PP branched polymer, and the reaction between MAPP and PLA. And, the rheological test results confirmed that a small amount of branched polymer could improve the melt elasticity of PLA/MAPP blends.

Meanwhile, the morphology of MAPP dispersion phase was observed by SEM. The PLA/MAPP blends with various MAPP contents were foamed under the same condition using supercritical CO₂ as blowing agent. Cells tended to nucleate in the interface of PLA/MAPP blends in foaming process. Thus, the PLA/MAPP dispersion phase morphology greatly affected the cellular structure parameters, especially the cell density. In addition, the PLA foam with higher expansion ratio and cell density can be obtained by adding proper MAPP content in the blends.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article: This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

References

Dorgan

Lehermeier

Palade

. Polylactides: properties and prospects of an environmentally benign plastic from renewable resources. J Macromol Sympos 2001; 175: 55–66.

Gewin

. Food safety – genetically modified corn. Comput Biol 2003; 13: 15–19.

Byun

Kim

Whiteside

. Characterization of an antioxidant polylactic acid (PLA) film prepared with α-tocopherol, BHT and polyethylene glycol using film cast extruder. J Food Eng 2010; 100: 239–244.

Garlotta

. A literature review of poly (lactic acid). J Polym Environ 2001; 100: 63–68.

Auras

Singh

. Performance evaluation of PLA against existing PET and PS containers. J Test Eval 2006; 34: 530–536.

Sawyer

. Bioprocessing – no longer a field of dreams. Macromol Sympos 2003; 201: 271–282.

Liu

Wang

. Study on rheological and extrusion foaming behaviors of chain-extended poly (lactic acid)/clay nanocomposites. J Cell Plast 2013; 49: 535–554.

Pavia

La Carrubba

Brucato

. Morphology and thermal properties of foams prepared via thermally induced phase separation based on polylactic acid blends. J Cell Plast 2012; 48: 399–407.

Cuénoud

Bourban

Plummer

CJG

. Plasticization of polylactide foams for tissue engineering. J Cell Plast 2012; 48: 409–432.

10.

Boissard

CIR

Bourban

Plummer

CJG

. Cellular biocomposites from polylactide and microfibrillated cellulose. J Cell Plast 2012; 48: 445–458.

11.

Wang

Jing

Cheng

. Blends of linear and long-chain branched poly(L-lactide)s with high melt strength and fast crystallization rate. Ind Eng Chem Res 2012; 51: 10088–10099.

12.

Iannace

Di Maio

. Reactively modified poly (lactic acid): properties and foam processing. J Macromol Mater Eng 2005; 290: 1083–1090.

13.

Liu

Wang

. Effect of chain extension on the rheological property and thermal behaviour of poly (lactic acid) foams. Cell Polym 2013; 32(6): 343–367.

14.

Wang

Gross

. Reactive compatibilization of biodegradable blends of poly(lactic acid) and poly(ɛ-caprolactone). Polym Degrad Stab 1998; 59: 161–168.

15.

Wang

Liu

Zhou

. Study on the effect of dispersion phase morphology on porous structure of poly (lactic acid)/poly (ethylene terephthalate glycol-modified) blending foams. Polymer 2013; 54(21): 5839–5851.

16.

De Roover

Sclavons

Carlier

. Molecular characterization of maleic anhydride‐functionalized polypropylene. J Polym Sci Part A Polym Chem 1995; 33(5): 829–842.

17.

Japon

Boogh

Leferrier

. Reactive processing of poly(ethylene terephthalate) modified with multifunctional epoxy-based additive. Polymer 2000; 41: 5809–5818.

18.

Upadhyay

Mohanty

Nayak

. Impact modification of poly (trimethylene terephthalate)/polypropylene blend nanocomposites: fabrication and characterization. J Appl Polym Sci 2011; 120: 932–943.

19.

Marco

Ellis

Gomez

. Rheological properties, crystallization, and morphology of compatibilized blends of isotactic polypropylene and polyamide. J Appl Polym Sci 1997; 65: 2665–2677.

20.

Kim

Lee

Choi

. The effect of types of maleic anhydride-grafted polypropylene (MAPP) on the interfacial adhesion properties of bio-flour-filled polypropylene composites. Compos Part A Appl Sci Manuf 2007; 38: 1473–1482.

21.

Garancher

Fernyhough

. Crystallinity effects in polylactic acid-based foams. J Cell Plast 2012; 48: 387–397.

22.

Zhai

Zhu

. A study of the crystallization, melting, and foaming behaviors of polylactic acid in compressed CO₂. Int J Mol Sci 2009; 10: 5381–5397.

23.

Lee ST, Park CB and Ramesh NS. Polymeric foams: science and technology. vol. 3. Florida, United States: CRC press, 2006.

24.

Takada

Hasegawa

Ohshima

. Crystallization kinetics of poly(L-lactide) in contact with pressurized CO₂. Polym Eng Sci 2004; 44: 186–196.

25.

Mihai

Huneault

Favis

. Crystallinity development in cellular poly(lactic acid) in the presence of supercritical carbon dioxide. J Appl Polym Sci 2009; 113: 2920–2932.

26.

Fischer

Sterzel

Wegner

. Investigation of the structure of solution grown crystals of lactide copolymers by means of chemical reactions. Coll Polym Sci 1973; 251: 980–990.

27.

Meng

Heuzey

Carreau

. Control of thermal degradation of polylactide/clay nanocomposites during melt processing by chain extension reaction. Polym Degrad Stab 2012; 97: 2010–2020.

28.

Wang

Jing

Huang

. Blends of linear and long-chain branched poly(L-lactide)s with high melt strength and fast crystallization rate. Ind Eng Chem Res 2012; 51: 10088–10099.

29.

Wang

Zhu

Zhang

. Continuous processing of low-density, microcellular poly (lactic acid) foams with controlled cell morphology and crystallinity. Chem Eng Sci 2012; 75: 390–399.

30.

Zhang

Liao

. Flow induced crystallization of long chain branched polypropylenes under weak shear flow. Eur Polym J 2009; 45: 2110–2118.

31.

Hingmann

Marczinke

. Shear and elongational flow properties of polypropylene melts. J Rheol 1994; 38: 573–588.

32.

Naguib

Park

Reichelt

. Strategies for achieving ultra low-density polypropylene foams. Polym Eng Sci 2002; 42: 1481–1492.

33.

Hartland S (ed.) Surface and interfacial tension: measurement, theory, and applications. vol. 119. Florida, United States: CRC press, 2004.

Role of maleic- anhydride-grafted- polypropylene in supercritical CO 2 foaming of poly (lactic acid) and its effect on cellular morphology

Abstract

Keywords

Introduction

Materials and methods

Materials

Preparation of PLA/MAPP blends

Supercritical CO2 batch foaming

Characterizations

Differential scanning calorimetry (DSC)

Polarizing optical microscope (POM)

X-ray diffraction (XRD)

Fourier transformation infrared spectroscopy (FTIR)

Shear rheometry

Foaming properties

Results and discussion

Reaction between PLA and MAPP

Thermal behavior of PLA/MAPP blends

Rheological behavior of PLA/MAPP blends

Morphology of PLA/MAPP blends and foams

Conclusions

Footnotes

Declaration of Conflicting Interests

Funding

References

Supercritical CO₂ batch foaming