Effect of stereocomplexation on the microcellular foaming behaviour at high temperature, compressive property and heat resistance of branched poly(L-lactide)/poly(D-lactide)

doi:10.21203/rs.3.rs-4985487/v1

To improve the melt strength and crystallisation property of polylactic acid (PLA) for achieving a good foaming performance at high temperatures, linear poly(l-lactide) (PLLA) was mixed with an epoxy chain extender to obtain branched PLLA (bPLLA), which was then blended with poly(d-lactide) (PDLA) to prepare a bPLLA/PDLA blend. The bPLLA/PDLA blend produced stereocomplex (SC) crystals, which increased the melt strength and viscosity of the blend. The synergistic effect of the SC crystals and the branched structure endowed bPLLA/PDLA with high melt strength and processability. In contrast, the nucleation effect of the SC crystals on bPLLA reduced the cell size, resulting in excellent microcellular foamability at the melting temperature. The formation of SC crystals in the blending process increased the crystallinity and enhanced the cell structure. As a result, the compressive strength of bPLLA/PDLA is increased from 0.44 MPa to 0.72 MPa. At 150°C, the dimensional deformation rate decreased from 42.59–13.13%, whereas the heat resistance increased by > 300%. This research provides a facile method for preparing PLA microcellular foams with high performance at high temperatures, which is essential for practical applications.

polylactic acid foams

stereocomplex crystals

compressive property

heat resistance

To alleviate the environmental and resource problems caused by the excessive use of fossil fuels, researchers have focused on bio-derived and biodegradable polymers as alternatives to traditional petroleum-based plastics^1–4. For instance, polylactic acid (PLA) is predominantly used to replace plastic owing to its excellent degradability, biocompatibility, rigidity and processability^5–6. PLA microcellular foams are lightweight, high-performance foams that are primarily used in various fields such as packaging, transportation, pharmaceutical industries as well as specialised materials such as scaffolds for human tissues⁷. To ensure continuous production of PLA foams, preparation of high-performance PLA microcellular foams under high-temperature conditions is essential. However, the poor melt strength of linear PLA results in poor foaming performance at high temperatures^8–13. In addition, PLA suffers from slow crystallisation and low crystallinity, leading to insufficient heat resistance and poor mechanical property^14,15.

Research attempting to improve the melt strength of PLA include crystallisation control¹⁶ and chain extension modification^17–19. Zhou et al.²⁰ improved the melt strength of PLA by adding a chain extender and prepared a foam with higher cell density. Reportedly, low-density PLA microcellular foams can be prepared based on the synergistic effect of crystallisation and a branched structure in the foaming process Tang et al.²¹ added a self-assembled nucleating agent to branched PLLA for adjusting its crystallisation behaviour, obtaining a low-density microcellular foams. However, when the foaming temperature exceeds the melting temperature of homo-crystallites (HCs)—which are formed during the cooling process of PLA—the cell of the PLA foam breaks and collapses, hindering a continuous foaming. Conversely, during the melt blending of poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA), an eutectic phenomenon leading to the formation of stereocomplex (SC) crystals occurs. The melting point of SC-PLA is approximately 210°C, which is higher than that of HC-PLA by around 50°C^22–24. During melt blending, the formation of SC crystals endows PLA with high melt strength at high temperature²⁵. The PLA system induced by SC-PLA formation is known to contain micro-cross-linked structures^26,27, in which the SC-PLA microcrystalline region serves as a physical cross-linking point for PLA molecular chains²⁸, endowing PLA with excellent mechanical properties and good heat resistance^29–31. In addition, the micro-cross-linked PLA system affects the foaming behaviour. Jia et al.³² introduced SC crystals into PLLA foams to obtain a PLLA/PDLA foam with high cell density and a highly uniform cell structure. Unfortunately, the continuous extrusion foaming of PLLA/PDLA foams is difficult to realise in industrial production. Yan et al.³³ prepared PLLA/PDLA foam with high expansion rate using a melt foaming method. However, the foaming temperature must be low to prevent the foam cell from cracking, and the development of a preparation method for microcellular foams at high temperature is still required. Considerably, introducing SC crystals can serve as nucleation sites for cells and enhance the melt strength, producing microporous foam.

Herein, branched PLA was prepared by blending PLA with a multi-functional epoxy chain extender. PDLA was introduced to branched PLLA (bPLLA) to prepare bPLLA/PDLA blends, which were foamed at the molten state via supercritical CO₂ batch foaming. Differential scanning calorimetry (DSC) and rheological measurements were performed to analyse the effect of SC crystals on the crystallisation and rheological properties of the bPLLA/PDLA blends. Additionally, the effect of the SC crystals on the foaming behaviour of bPLLA at high temperatures was studied. Finally, the heat resistance and compression properties of the resulting bPLLA/PDLA foams were tested.

2.1. Materials

PLLA (2003D, Mw = 215 kg/mol, PDI = 1.7) was purchased from Nature Works LLC. PDLA (ρ = 1.24 g/cm³, MFI = 12 g/10 min) was supplied by Corbion. The ADR4468 chain extender (Mw = 7.25 kg/mol), which is a copolymer containing epoxy functional groups with nine active groups, was purchased from BASF. CO₂ with a purity of 99.99% was provided by Beijing Oriental Medical Gas Co. Ltd., China.

2.2. Preparation of bPLLA /PDLA blends

Before blending, PLLA and PDLA were dried in a vacuum oven at 80°C for 6 h, and ADR 4468 was put into a vacuum oven at 50°C for 6 h for removing water. A series of bPLLA/PDLA blends with different PDLA contents were prepared via melt blending in a HAAKE mixer (XSS-300). First, PLLA was mixed with 2.5% ADR at 190°C for 5 min at 60 rpm to prepare bPLLA. Subsequently, different contents of PDLA were added and the mixtures were allowed to blend for 10 min, affording the bPDLA/PDLA5, bPLLA/PDLA10, bPLLA/PDLA15 and bPLLA/PDLA20 blends. The obtained samples were pressed into thin slices with dimensions of 10 cm × 10 cm × 1 mm at 190°C and 5 MPa for 5 min. The proportions of the blends are listed in Table 1.

Table 1

**Composition of the bPLLA/PDLA blends**
Sample	bPLLA/phr	PDLA/phr
bPLLA	100	0
bPLLA/PDLA5	95	5
bPLLA /PDLA10	90	10
bPLLA /PDLA15	85	15
bPLLA /PDLA20	80	20

2.3. Preparation of bPLLA and bPLLA/PDLA foams

Autoclaves were used for batch foaming to control the foaming process. The preparation process is shown in Fig. 1. First, samples with dimensions of 1 cm × 2 cm × 1 mm were put into the autoclave and heated to 180°C. Simultaneously, the high-pressure vessel was pressurised with supercritical CO₂. To study the foaming behaviour of bPLLA/PDLA at different temperatures, the temperature of the autoclave was decreased from 180°C to 120°C, 130°C, 140°C and 150°C. Subsequently, the pressure was relieved after isothermal treatment for 20 min and maintained at 15 MPa during the whole process using a pump.

2.4. Characterisations

2.4.1. Crystallisation behaviour

First, the thermal performance of the samples was tested at atmospheric pressure using DSC (TA instrument, DSC-Q100) under nitrogen conditions. Briefly, 5–10 mg of the sample was put into a crucible and quickly heat to 190°C for 3 min to eliminate the thermal history. Next, the crystallisation behaviour of the samples was studied by cooling to 30°C at 10°C/min for 3 min, followed by heating to 240°C at 10°C/min to study the melting behaviour. Finally, the sample was cooled to 30°C at = 10°C/min.

To study the crystallisation behaviour of the foam, 5–10 mg of foam samples were put into a crucible and heated to 240°C at 10°C/min to study the melting behaviour. The HC and SC crystallinity was determined using Eqs. 1 and 2, respectively,

$$\:{\chi\:}_{c\left(HC\right)}=\frac{{\varDelta\:H}_{m\left(HC\right)}-\varDelta\:{H}_{c\left(HC\right)}}{{\varDelta\:H}_{m\left(HC\right)}^{0}}$$

1

$$\:{\chi\:}_{c\left(SC\right)}=\frac{{\varDelta\:H}_{m\left(SC\right)}}{{\varDelta\:H}_{m\left(HC\right)}^{0}\times\:\omega\:}$$

2

,

where ΔH_c(HC) is the cold crystallisation enthalpy of PLA, ΔH_m(HC) is the melting enthalpy of PLA, ΔH_m(SC) is the melting enthalpy of SC, ΔH⁰_m(HC) is the standard enthalpy of fusion—with a value of 93.6 J/g³⁴—and ω is the mass fraction of PLLA (%).

2.4.2. Rheological characterisation

The rheological behaviour of bPLLA/PDLA was characterised using a rotational rheometer (HAAKE MARS rheometer, TA, USA). Samples for measurement were made into a circular plate with a diameter of 25 mm and a thickness of 1 mm. To remain in the linear viscoelastic region, the amplitude was 5%. The rheological property was tested at 190°C at an angular velocity of 0.1–100 rad/s.

2.4.3. Cell morphology characterisation

To observe the foam structures in detail, the foam samples were soaked in liquid nitrogen for 2 h and then brittle fractured. Au was sputtered on the sample surface using a spraying equipment (Germany, K550X, Germany, Czech Republic). Subsequently, the brittle fracture section of the foam was observed via scanning electron microscopy (SEM, Regulus 8100, Hitachi, Japan). Cell data were calculated using Image Pro Plus 6.0, and the cell density was determined according to Eqs. 3 and 4:

$$\:n={\left(\frac{{n}_{b}}{{L}^{2}}\right)}^{\frac{3}{2}}\times\:\text{Φ}$$

3

$$\:\text{Φ=}\frac{\rho\:}{{\rho\:}_{f}}$$

4

,

where n_b is the number of cells in the statistical area, L² is the cell surface area (cm²), Φ is the expansion ratio, ρ_f is the density of the foamed sample (g/cm³) and ρ is the density of the unfoamed sample (g/cm³).

2.4.4. Thermal performance characterisation

Heat deformation temperature (HDT)

To investigate the heat resistance of the bPLLA/PDLA blend, the HDT was measured using a heat deformation tester (ZWK1302-B, MTS SYSTEMS Co., Ltd., USA). Initially, a sample with a volume of 80 × 10 × 4 mm³ was placed on a Charpy structure with a span of 64 mm and then immersed in silicone oil. The central stress of the bPLLA/PDLA blend was 0.45 MPa. The heating rate of the silicone oil was 2°C/min. The temperature at which the deformation displacement of the bPLLA/PDLA blend reached 0.34 mm was considered the HDT. The test was conducted in accordance with the GB/T1634-2004 standard.

Dynamic mechanical analysis

The thermodynamic mechanical properties of the specimens (25 × 10 × 2 mm³) were measured using a dynamic mechanical analyser (TA Q800) under the single cantilever beam mode. The heating rate, frequency and amplitude were 3 K/min, 1 Hz and 15 µm, respectively.

Heat resistance characterisation

Foam samples were placed in an oven and observed for 10 min. The oven was pre-heated to different temperatures (140°C, 145°C and 150°C). The change in the foam samples thickness after the heating treatment was recorded to assess the thermal stability. The expansion ratio of the foam was 17 times (error: ±8%).

2.4.5. Mechanical property analysis

The compressive properties of bPLLA/PDLA foam samples with 10 × 10× 10 mm³ dimensions were measured using a universal tester. To eliminate the impact of the skin layer on the foam, all samples were subjected to peeling. The compression speed was 1 mm/min. The experiment ended at 80% strain. The compression test was repeated three times for each type of sample.

3.1. Effect of PDLA on the thermal behaviour of bPLLA/PDLA blends

Table 2

Thermal properties of all samples
Sample	ΔH_c−cold(Hc)/J/g	ΔH_m(HC)/J/g	ΔH_m(SC)/J/g	T_m(HC)/°C	T_m(SC) /°C	X_c(HC)/%	X_c(SC)/%
bPLLA	14.39	19.34	/	150.67	/	5.29	/
bPLLA/PDLA5	13.40	16.27	7.07	149.64	214.69	3.07	7.95
bPLLA/PDLA10	13.08	13.36	14.92	147.03	217.55	0.32	17.71
bPLLA/PDLA15	-	1.84	26.73	146.58	220.06	1.97	33.60
bPLLA/PDLA20	-	0.18	32.71	143.48	220.89	0.19	43.68

Figure 2a shows the cooling curve after eliminating the thermal history. There was no crystallisation peak during the cooling process, indicating the absence of HC formation during the cooling process.

Figure 2b shows the heating curve for the ramp up to 240°C with a melting point peak appearing near 220°C, indicating the formation of SC crystals³⁵. Table 2 shows that the crystallinity of SC crystals increased from 7.95–43.68% with increasing PDLA content. Due to the hydrogen bonding between PLLA and PDLA in the SC crystals, the PDLA content increases, further increasing the nucleation points of SC crystals and their crystallinity³⁶. In addition, the cold crystallisation peak of HCs gradually disappeared, whereas the melting peak gradually decreased due to the competition between SC crystal formation and HC formation gradually shifting from the latter to the former. Figure 2c shows the bPLLA/PDLA cooling curve. When the PDLA content was 20 phr, a crystallisation peak appeared because the formation of abundant SC crystals promotes the crystallinity of HCs²⁶. The DSC results indicate that the SC crystals can serve as nucleating agents for cells during foaming, thereby increasing the cell density. Moreover, the crystal network can enhance the melt strength. These two synergistic effects can produce high-density microporous foam. Additionally, an increase in crystallinity can enhance the heat resistance of PLLA.

3.2. Effect of stereocomplexation on the rheology and structure of the asymmetric bPLLA/PDLA blends

The viscoelastic response of the PDLA blend series was investigated by performing oscillatory shear rheology measurements at 190°C. When HCs melted at 190°C, SC crystals were still present in the blend owing to their higher melting point³⁷. The storage modulus (G’), complex viscosity (η*) and loss factor (tanδ) of the blends are shown in Fig. 3.

Figure 3a shows the change in the G' of the blends with the shear rate (ω). The G' value gradually increased with increasing PDLA content due to the increase in the number of SC crystals. The SC crystal zone formed a physical cross-linking network to increase the relaxation time of the chain segments³⁸, thereby improving the energy G’ of the blends. The increase in G’ suggests that the addition of PDLA improved the melt strength of the blends, which is conducive to the expansion of PLA foams^13,26.

Figure 3b shows that η* increased with increasing PDLA content. The increase in viscosity of the blends can promote the formation of smaller cells and reduce their rupture during cell expansion. Figure 3c shows that the tanδ value gradually approached 1 at 10 phr PDLA. As the SC crystallinity increased, SC crystals and molecular chains became interconnected, forming a class network structure^10,26,38. These results indicate that the bPLLA/PDLA blends have high melt strength and a network structure. This combination can improve the foaming performance, resulting in bPLLA/PDLA foams with smaller cell diameter and cell density.

3.3. Effect of stereocomplexation on the thermal performance

Figure 4 (a) shows that the HDT of the bPLLA/PDLA blend blends increased with increasing PDLA content. At a PDLA content of 20 phr, the bPLLA/PDLA blend exhibited the highest HDT of 62.4°C. This phenomenon can be explained by the formation of a physical cross-linking network between PLLA and PDLA, which improved the rigidity of PLA molecular chains and enhanced the heat resistance properties of the bPLLA/PDLA blend^39,40. The storage modulus (E′) of the samples as a function of temperature is shown in Fig. 4b. After adding PDLA, the storage modulus of the bPLLA/PDLA blend increased, indicating an improvement in the heat resistance. Subsequently, due to cold crystallisation of the matrix, the storage modulus started to increase and then decreased sharply. However, when the PDLA content was 20 phr, the storage modulus of the blend did not decrease remarkably. This can be attributed to the formation of a dense crystal network of bPLLA/PDLA through SC crystallisation, which imparts a high modulus to the blend at high temperatures and improves the heat resistance of the PLA matrix^9,41–43. In short, the heat resistance of the blends gradually increases as the PDLA content increases. The increase in heat resistance is beneficial for the practical applications of PLA materials.

3.4. Effect of stereocomplexation on the foaming behaviour at high temperature and cell structure of bPLLA/PDLA foam

Figure 5 shows SEM images of bPLLA/PDLA foam samples with different PDLA contents obtained at 130°C. Initially, the samples were put into the autoclave and heated to 180°C to melt the bPLLA matrix. Subsequently, the autoclave was cooled down to 130°C and held for 20 min. Finally, the pressure was released. When the foaming temperature was 130°C, the samples were in the molten state, verifying that the microcellular foam was generated at the melting temperature. The cell structure of bPLLA was severely ruptured owing to the low melt strength of bPLLA, which prevented the cell structure from withstanding the cell growth, resulting in the escape of CO₂ and the collapse and rupture of the cells^15,44,45. With increasing PDLA content, the cell structure remained intact and SC crystals were formed, enhancing the melt strength of the blend and its capacity to preserve and homogenise the cell structure.³² Furthermore, the SC crystals served as a nucleation point for cells, facilitating the formation of cells^2,46–49. The SC crystal content was high at a high PDLA content, and the cells were relatively intact even when the foaming temperature was increased to 130°C. When the PDLA content was 20 phr, SC crystals formed a physical network that inhibited the escape of CO₂, limiting cell growth. The decrease in melt strength of PLLA with increasing temperature led to cell rupture. At > 130°C, the cells of the bPLLA/PDLA20 foam began to expand due to the increased chain segment movement at higher temperatures, which promoted the diffusion of CO₂ and cell growth (Figure S1)²⁶.

Figure 6 displays the cell parameters of bPLLA/PDLA foams with different PDLA contents obtained at 130°C. The expansion ratio of the bPLLA/PDLA foam initially increased and then decreased with increasing PDLA content (Fig. 6a). Owing to its poor melt strength, bPLLA exhibited a high expansion ratio and its cells were ruptured easily. When PDLA was added and SC crystals were introduced, the melt strength of bPLLA/PDLA increased and the cell rupture decreased. As the PDLA content increased, the crystallinity of the SC crystals increased, preventing the cells from breaking and increasing the expansion ratio²⁶. Figure 6b shows that with increasing PDLA content, η* increased and the cell diameter decreased. Simultaneously, the physical network gradually strengthened and the cell structure was improved, thereby decreasing cell density. The formation of SC crystals as a nucleation point for cells promotes cell nucleation, and the foam gradually becomes a microcellular foam³³. The increase in PDLA content increased the SC crystallinity and the nucleation structure, facilitating the formation of cells and increasing the cell density during growth. Consequently, a more uniform distribution of bubbles was obtained (Fig. 6c)^33,40,47.

As shown in Fig. 7, cells with low PDLA content tend to collapse at a foaming temperature of 150°C. However, when the PDLA content was increased to 15 phr, the cells maintained a relatively intact structure with a notably smaller size of 5–20 µm (Figure S2). This phenomenon can be attributed to the higher PDLA content, promoting the formation of SC crystals and enhancing the system melt strength. This enhanced strength effectively restricts excessive cell growth, mitigates cell rupture and collapse, consequently forming smaller and more uniform cells^15,35,50.

3.5. Effect of stereocomplexation on the heat resistance of bPLLA/PDLA foams

Figure 8 shows photographs of the bPLLA/PDLA foam samples before and after the heat resistance test performed by recording the thickness of the samples before and after being loaded with 80 g weight and heated in an oven at different temperatures for 10 min.

Figure 9(a) shows the thickness changes of the foam samples before and after heat treatment at different temperatures (140°C, 145°C and 150°C). As shown in Fig. 10b, the bPLLA foam heat treated at 140°C underwent a thickness variation of 25.48%. The thickness variation decreased gradually as the PDLA content in the bPLLA/PDLA foam increased, until it reached 7.66% for the bPLLD/PDLA15 foam⁵¹. Meanwhile, the thickness variation of bPLLA increased with increasing temperature, reaching 42.59% at 150°C. These results reveal that the heat resistance of the bPLLA/PDLA foams increased considerably with increasing PDLA content. This can be attributed to the physical cross-linking network between PDLA and bPLLA, which increased the crystallinity of the bPLLA/PDLA blend, and the formation of SC crystals with higher melting point, which improved the strength of the foam at high temperature⁵².

3.6. Effect of stereocomplexation on the compressive properties of bPLLA/PDLA foams

Owing to the effect of foam density on the mechanical properties, foam with an expansion ratio of 17 times (error: ±8%) was used for the mechanical tests. Figure 10a displays the compressive stress–strain curves of the foam samples, indicating that the modulus and strength gradually increased with increasing PDLA content. As the PDLA content increases, the crystallinity of the SC crystals increases, forming a physical network. This enhances the ability of the bPLLA matrix to resist cell growth, limiting the cell growth and reducing the cell size. Consequently, stress disperses more uniformly in the samples when an external force is applied, improving the ability of the foam to resist deformation. The improvement in compressive modulus and strength due to the SC crystals may be due to their strong structure^{33, 47, 50}.

Figure 10b shows that the compression strength of bPLLA was 0.44 MPa when the strain was 10%. As the PDLA content increased, the compression modulus increased until reaching 0.72 MPa for the bPLLA/PDLA15 foam. The compression modulus of the bPLLA/PDLA foams increased by 164% compared with that of the bPLLA foam.

Figure 11 shows SEM images of ompressed foam samples with different PDLA contents. The SC crystals improved the ability to maintain and homogenise the cell structure. The size of the cells decreased, which resulted in a uniform stress distribution and improved the compression resistance of the foam.

Table 3

Thermal properties of compressed foam samples
Sample	ΔH_m(HC)/ J/g	ΔH_m(SC)/ J/g	Tm_(HC) /°C	Tm_(SC) /°C	Xc_(HC)/%	Xc_(SC)/%
bPLLA	14.94	/	162.41	/	15.96	/
bPLLA/PDLA5	18.47	3.71	150.34	214.42	19.73	6.45
bPLLA /PDLA10	19.36	9.04	157.52	221.68	20.68	18.52
bPLLA /PDLA15	7.76	16.50	158.28	224.9	8.29	25.25

Figure 12 shows the heating curves of bPLLA/PDLA foams. From Fig. 12 and Table 3, the HC crystallinity of the foams increased with increasing PDLA content because adding PDLA promoted the formation of SC crystals. Moreover, for the bPLLA/PDLA15 foam, the higher number of nucleation sites of SC crystals facilitates the formation of more SC crystals, resulting in higher strength³⁸.

bPLLA was obtained by blending PLLA with a chain extender, and PDLA was introduced to prepare bPLLA/PDLA blends. The formation of SC crystals in the bPLLA/PDLA blends increased the melt strength and viscosity of the blends. The synergistic effect of the SC crystals and the branched structure endowed the bPLLA/PDLA blends with high melt strength and processability, resulting in excellent microcellular foamability at high temperatures. At a foaming temperature of 150°C, the blends with high PDLA content maintained a relatively complete cell structure and low cell size. In addition, due to the formation of the SC crystals, the compression strength of bPLLA/PDLA is increased from 0.44 MPa to 0.72 MPa. At 150°C compared with that of bPLLA and the heat resistance at 150°C increased by 300%.

Conflicts of interest

No known competitive economic interests or personal relationships

Author Contribution

Z.M.X wrote the main manuscript text . L.S provided experimental methodology. C.S.H project administration, Supervision. W.X.D provided concepts, formal analysis, project management, resources, and supervision. W.Y.Q review, edit, and modify the original manuscript

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No competing interests reported.

Supportinginformation.doc

Effect of stereocomplexation on the microcellular foaming behaviour at high temperature, compressive property and heat resistance of branched poly(L-lactide)/poly(D-lactide)

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Version 1

Abstract

Figures

1. Introduction

2. Materials and methods