Designing Multiple-Shape Memory Polymers with Miscible Polymer Blends: Evidence and Origins of a Triple-Shape Memory Effect for Miscible PLLA/PMMA Blends

Shape memory properties of polymers represent one of the most expanding fields in polymer science related to numerous smart applications. Recently, multiple-shape memory polymers (multiple-SMPs) have attracted significant attention and can be achieved with complex polymer architectures. Here, miscible PLLA/PMMA blends with broad glass transitions are investigated as an alternative platform to design multiple-SMPs. Dual-shape memory experiments were first conducted at different stretching temperatures to identify the so-called “temperature memory effect”. The switch temperature of the symmetric 50% PLLA/50% PMMA blend smoothly shifted from 70 to 90 °C for stretching temperatures increasing from 65 to 94 °C, attesting for a significant “temperature memory effect”. Asymmetric formulations with 30% and 80% PMMA also present a “temperature memory effect”, but the symmetric blend clearly appeared as the most efficient formulation for multiple-shape memory applications. A programming step designed with two successive stretchings within the broad glass transition consequently afforded high triple-shape memory performances with tunable intermediate shapes, demonstrating that the symmetric blend could represent an interesting candidate for future developments. Advanced shape recovery processes are consistent with a selective activation of specific “soft domains” or nanodomains arising from the broad glass transition and the large distribution of relaxation time observed by DSC and dielectric spectroscopy. Polarized IR measurements pointed out that the composition of activated/oriented “soft domains” could vary with stretching temperature, giving rise to the “temperature-memory effect”. Consequently, from a polymer physics standpoint, nanoscale compositional heterogeneities within the symmetric blend could be suspected and discussed on the basis of available models for miscible blends and for multiple-SMPs

Engineered polylactide (PLA)–polyamide (PA) blends for durable applications: 1. PLA with high crystallization ability to tune up the properties of PLA/PA12 blends

Polylactide (PLA), a biodegradable polyester produced from renewable resources, has a key position in the very promising market for bioplastics. Unfortunately, for utilization in durable/engineering applications, PLA suffers from some shortcomings such as low rate of crystallization, brittleness, and small ductility. The study proposes the use of PLA having high crystallization ability to tune up the properties of partly bio-based PLA/polyamide 12 (PA12) blends in presence of key additives. First, phenylphosphonic acid zinc salt (PPA-Zn) was selected as one of the most adapted nucleating agents (NAs) for PLA, whereas larger quantities of PLA(NA) have been produced for blending with PA12. The characterizations of PLA(NA) confirm dramatic improvements of PLA crystallization kinetics and an impressive degree of crystallinity (>40%). Blends having different PLA(NA)/PA12 ratios were prepared by melt-mixing with a laboratory micro-­compounder and characterized in terms of morphology, thermal stability, and with focus on the evidence of advanced crystallization properties. All differential scanning calorimetry measurements of PLA(NA)/PA12 blends suggest powerful nucleation and crystallization ability. Furthermore, addition of epoxy-functional styrene-acrylic compatibilizers into selected compositions by reactive extrusion (REX) was found to significantly change their morphology, preserving the properties of crystallization of PLA, with enhancements of mechanical properties (strength, ductility, impact resistance) confirmed by current prospects.</p

Stereocomplexation of Polylactide Enhanced by Poly(methyl methacrylate): Improved Processability and Thermomechanical Properties of Stereocomplexable Polylactide-Based Materials

Stereocomplexable polylactides (PLAs) with improved processability and thermomechanical properties have been prepared by one-step melt blending of high-molecular-weight poly­(l-lactide) (PLLA), poly­(d-lactide) (PDLA), and poly­(methyl methacrylate) (PMMA). Crystallization of PLA stereocomplexes occurred during cooling from the melt, and, surprisingly, PMMA enhanced the amount of stereocomplex formation, especially with the addition of 30–40 % PMMA. The prepared ternary blends were found to be miscible, and such miscibility is likely a key factor to the role of PMMA in enhancing stereocomplexation. In addition, the incorporation of PMMA during compounding substantially raised the melt viscosity at 230 °C. Therefore, to some extent, the use of PMMA could also overcome processing difficulties associated with low viscosities of stereocomplexable PLA-based materials. Semicrystalline miscible blends with good transparency were recovered after injection molding, and in a first approach, the thermomechanical properties could be tuned by the PMMA content. Superior storage modulus and thermal resistance to deformation were thereby found for semicrystalline ternary blends compared to binary PLLA/PMMA blends. The amount of PLA stereocomplexes could be significantly increased with an additional thermal treatment, without compromising transparency. This could result in a remarkable thermal resistance to deformation at much higher temperatures than with conventional PLA. Consequently, stereocomplex crystallization into miscible PLLA/PDLA/PMMA blends represents a relevant approach to developing transparent, heat-resistant, and partly biobased polymers using conventional injection-molding processes

Toward “Green” Hybrid Materials: Core–Shell Particles with Enhanced Impact Energy Absorbing Ability

Restrained properties of “green” degradable products drive the creation of materials with innovative structures and retained eco-attributes. Herein, we introduce the creation of impact modifiers in the form of core–shell (CS) particles toward the creation of “green” composite materials. Particles with CS structure constituted of PLA stereocomplex (PLASC) and a rubbery phase of poly­(ε-caprolactone-<i>co</i>-d,l-lactide) (P­[CL-<i>co</i>-LA]) were successfully achieved by spray droplet atomization. A synergistic association of the soft P­[CL-<i>co</i>-LA] and hard PLASC domains in the core–shell structure induced unique thermo-mechanical effects on the PLA-based composites. The core–shell particles enhanced the crystallization of PLA matrices by acting as nucleating agents. The core–shell particles functioned efficiently as impact modifiers with minimal effect on the composites stiffness and strength. These findings provide a new platform for scalable design of polymeric-based structures to be used in the creation of advanced degradable materials

Green and Efficient Synthesis of Dispersible Cellulose Nanocrystals in Biobased Polyesters for Engineering Applications

Despite attractive properties of cellulose nanocrystals (CNCs) such as high natural abundance, inherent biodegradability and high modulus, CNCs tend to degrade and aggregate when exposed to high temperatures during melt processing. In the present work, the surface of CNCs was modified with PMMA to take advantage of the miscibility with various biobased polymers including PLLA when melt-blended. Particular attention was paid to grafting techniques in water medium using two different redox initiators: Fe²⁺/H₂O₂ (Fenton’s reagent) and ceric ammonium nitrate (CAN). The successful synthesis of CNC-<i>g</i>-PMMA was verified by gravimetric analysis, FTIR, CP-MAS ¹³C NMR and suspension tests. A high grafting efficiency of 77% was achieved using CAN as the redox initiator. Increasing the PMMA content on CNC surfaces led to higher CNC thermal stability. As a consequence of PMMA grafting in water, modified CNCs were found to be predispersed in a PMMA network. PLLA/CNC nanocomposites were then prepared by melt-blending, i.e., in the absence of solvent, and the quality of the dispersion was confirmed by dynamic rheology, TEM and DMA. The presence of a high amount of PMMA grafts on CNC surfaces reduced CNC aggregation and favors the percolation of CNCs with the development of a weak long-range 3D network. Miscibility between PMMA grafts and PLLA as well as the predispersion of CNCs was found to play a key role in the dispersion of CNCs in PLLA. Thermomechanical analysis revealed that PMMA grafts on CNC surfaces significantly enhanced elastic moduli in the glassy and rubbery state. The high dispersion state (related to high PMMA grafting) also showed a positive effect on O₂ permeability of PLLA and a strong beneficial effect on heat deflection temperature (HDT) reaching outstanding temperatures higher than 130 °C. Thus, free-radical grafting of PMMA in water provides an efficient and green route to dispersible (bio)­nanofillers by solvent-free extrusion techniques with PMMA-miscible matrices such as PLLA for high-performance applications

Recent advances in production of poly(lactic acid) (PLA) nanocomposites: a versatile method to tune crystallization properties of PLA

<p>A new approach leading to poly(lactic acid) (PLA) nanocomposites designed with improved nucleating/crystallization ability has been developed. As proof of concept, nanofillers of different morphology (organo-modified layered silicates, halloysite nanotubes and silica) were surface-treated with ethylene bis-stearamide (EBS), a selected fatty amide able to promote chain mobility during PLA crystallization from the melt and nucleation. The fine dispersion of the nucleating additive via nanoparticles (NPs) as ‘nano-template’ is leading to nanocomposites showing unexpected improvements in PLA crystallization rate. This was evidenced by differential scanning calorimetry (DSC) from the high values of the degree of crystallinity (20–40%) with respect to neat PLA (4.3%) and the sharp decrease in crystallization half-time under isothermal conditions (at 110°C), even below one minute. Furthermore, after injection molding the outstanding crystallization properties of PLA were again confirmed. Accordingly, the PLA-nanofiller/EBS nanocomposites revealed remarkable degree of crystallinity (in the range of 30–40%). Surprisingly, the presence of EBS can significantly increase the impact resistance of PLA and PLA based nanocomposites. By considering the remarkable increasing in crystallinity, a key parameter to allow PLA utilization in durable applications, the development of the new approach is expected to lead to significant improvements in the processing and performances of PLA products.</p

Preparation of Cellulose Nanocrystal-Reinforced Poly(lactic acid) Nanocomposites through Noncovalent Modification with PLLA-Based Surfactants

Cellulose nanocrystal (CNC)-reinforced poly­(lactic acid) (PLA) nanocomposites were prepared by twin-screw extrusion followed by injection-molding using a masterbatch approach. Noncovalent modification of CNCs was performed with two different poly­(l-lactide) (PLLA)-based surfactants to improve the filler/matrix compatibility. They both have a PLLA block that is expected to improve the compatibility with the PLA matrix and differ by the polar head. It consists of either a poly­(ethylene glycol) (PEG) block (PEG-<i>b</i>-PLLA) or an imidazolium group (Im-PLLA), that is able to interact with the surface of the CNCs. The morphological, structural, thermal, rheological, and mechanical properties of the nanocomposites were investigated. The different modes of interaction of the polar head of the surfactant lead to different properties. However, the global decrease in the molecular weight of PLA, induced by the short PLLA blocks from the surfactants and the possible degradation during melt processing, results in a plasticization effect and impacts the crystallization of the matrix

Shape-Memory Behavior of Polylactide/Silica Ionic Hybrids

Commercial polylactide (PLA) was converted and endowed with shape-memory properties by synthesizing ionic hybrids based on blends of PLA with imidazolium-terminated PLA and poly­[ε-caprolactone-<i>co</i>-d,l-lactide] (P­[CL-<i>co</i>-LA]) and surface-modified silica nanoparticles. The electrostatic interactions assist with the silica nanoparticle dispersion in the polymer matrix. Since nanoparticle dispersion in polymers is a perennial challenge and has prevented nanocomposites from reaching their full potential in terms of performance we expect this new design will be exploited in other polymers systems to synthesize well-dispersed nanocomposites. Rheological measurements of the ionic hybrids are consistent with the formation of a network. The ionic hybrids are also much more deformable compared to the neat PLA. More importantly, they exhibit shape-memory behavior with fixity ratio <i>R</i>_f ≈ 100% and recovery ratio <i>R</i>_r = 79%, for the blend containing 25 wt % <i>im</i>-PLA and 25 wt % <i>im</i>-P­[CL-<i>co</i>-LA] and 5 wt % of SiO₂–SO₃Na. Dielectric spectroscopy and dynamic mechanical analysis show a second, low-frequency relaxation attributed to strongly immobilized polymer chains on silica due to electrostatic interactions. Creep compliance tests further suggest that the ionic interactions prevent permanent slippage in the hybrids which is most likely responsible for the significant shape-memory behavior observed

Mechanistic insights on nanosilica self-networking inducing ultra-toughness of rubber-modified polylactide-based materials

<p>Developing novel strategies to improve the impact strength of PLA-based materials is gaining a significant importance in order to enlarge the range of applications for this renewable polymer. Recently, the authors have designed ultra-tough polylactide (PLA)-based materials through co-addition of rubber-like poly(ϵ-caprolactone-<i>co</i>-d,l-lactide) (P[CL-<i>co</i>-LA]) impact modifier and silica nanoparticles (SiO₂) using extrusion techniques. The addition of silica nanoparticles into these immiscible PLA/P[CL-<i>co</i>-LA] blends altered their final morphology, changing it from rubbery spherical inclusions to almost oblong structures. A synergistic toughening effect of the combination of P[CL-<i>co</i>-LA] copolymer and silica nanoparticles on the resulting PLA-based materials therefore occurred. To explain this particular behavior, the present work hence aims at establishing the mechanistic features about the nanoparticle-induced impact enhancement in these immiscible PLA/impact modifier blends. Incorporation of silica nanoparticles of different surface treatments and sizes was thereby investigated by means of rheological, mechanical and morphological methods in order to highlight the key parameters responsible for the final impact performances of the as-produced PLA-based materials. Relying on video-controlled tensile testing experiments, a toughening mechanism was finally proposed to account for the impact behavior of resulting nanocomposites.</p

Multiresponsive Shape Memory Blends and Nanocomposites Based on Starch

Smart multiresponsive bionanocomposites with both humidity- and thermally activated shape-memory effects, based on blends of ethylene-vinyl acetate (EVA) and thermoplastic starch (TPS) are designed. Thermo- and humidity-mechanical cyclic experiments are performed in order to demonstrate the humidity- as well as thermally activated shape memory properties of the starch-based materials. In particular, the induced-crystallization is used in order to thermally activate the EVA shape memory response. The shape memory results of both blends and their nanocomposites reflect the excellent ability to both humidity- and thermally activated recover of the initial shape with values higher than 80 and 90%, respectively