15 research outputs found
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 âtemperature memory effectâ.
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 âtemperature
memory effectâ. Asymmetric formulations with 30% and 80% PMMA
also present a âtemperature memory effectâ, 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 âsoft domainsâ 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 âsoft
domainsâ could vary with stretching temperature, giving rise
to the âtemperature-memory effectâ. 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)âpolyamide (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-Âcompounder 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
polyÂ(l-lactide) (PLLA), polyÂ(d-lactide) (PDLA),
and polyÂ(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 30â40 % 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 âGreenâ Hybrid Materials: CoreâShell Particles with Enhanced Impact Energy Absorbing Ability
Restrained
properties of âgreenâ 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 coreâshell (CS) particles toward the creation
of âgreenâ composite materials. Particles with CS structure
constituted of PLA stereocomplex (PLASC) and a rubbery phase of polyÂ(Δ-caprolactone-<i>co</i>-d,l-lactide) (PÂ[CL-<i>co</i>-LA])
were successfully achieved by spray droplet atomization. A synergistic
association of the soft PÂ[CL-<i>co</i>-LA] and hard PLASC
domains in the coreâshell structure induced unique thermo-mechanical
effects on the PLA-based composites. The coreâshell particles
enhanced the crystallization of PLA matrices by acting as nucleating
agents. The coreâshell 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: Fe<sup>2+</sup>/H<sub>2</sub>O<sub>2</sub> (Fentonâs
reagent) and ceric ammonium nitrate (CAN). The successful synthesis
of CNC-<i>g</i>-PMMA was verified by gravimetric analysis,
FTIR, CP-MAS <sup>13</sup>C 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 O<sub>2</sub> 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)Ânanofillers
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 ânano-templateâ 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 (20â40%) 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 30â40%). 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 polyÂ(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 polyÂ(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 polyÂ(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 polyÂ[Δ-caprolactone-<i>co</i>-d,l-lactide] (PÂ[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><sub>f</sub> â
100% and recovery ratio <i>R</i><sub>r</sub> = 79%, for
the blend containing 25 wt % <i>im</i>-PLA and 25 wt % <i>im</i>-PÂ[CL-<i>co</i>-LA] and 5 wt % of SiO<sub>2</sub>âSO<sub>3</sub>Na. 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(Ï”-caprolactone-<i>co</i>-d,l-lactide) (P[CL-<i>co</i>-LA]) impact modifier and silica nanoparticles (SiO<sub>2</sub>) 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