Structural and dynamic heterogeneity in the amorphous phase of PLLA confined at the nanoscale by co-extrusion process

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Literature Review

Poly(lactide) (PLA) polymers are well known for their biodegradability and especially by the fact that they are made from renewable resources as corn starch or potato which makes them « green » polymers. Several studies have showed that PLA is potentially usable in some packaging sectors [1,2] and the concern with the environment has increased the interest in the development of biodegradable packaging materials to replace plastic packaging coming from the petrochemical industry. The materials used in packaging must have good gas barrier properties to increase their ranges of use, particularly in the food, cosmetics and medicine. Through recycling, marketing and costs, metal and glass packaging were replaced by polymers, whose barrier properties are not well known and hence the interest from the study in order to improve it. The gas barrier properties of a polymer are not only linked to the crystalline phase (content, orientation) but also to the mobility of the amorphous phase. An alternative strategy to change properties of polymers, mainly permeability, is to change the length scale attained by plastics processing to the macromolecular scale. Confinement of the polymers can create new amorphous and/or crystalline microstructures, which can lead to a significant improvement of properties
In this chapter, we will first of all discuss about the influence of crystallinity and amorphous phase on barrier properties of bulk PLA, the structure/function relationship between PLA microstructure and transport properties and then the impact of confined structures on crystallinity, amorphous phase mobility and consequently on permeability.

Characterization of PLA

Synthesis of PLA

PLA can be synthesized by different routes, including two paths which allow to obtain high molecular weight polymers: the polycondensation of lactic acid (often using coupling agents for increasing the molecular weight) and polymerization by opening the lactide ring, obtained by condensation of two lactic acids [3]. The latter process is used in majority to produce commercially available PLA. Lactic acid (2-hydroxypropanoic acid) is obtained by fermentation of glucose coming from vegetal sources. It has a chiral carbon atom and exists in two enantiomeric forms called L-lactic acid and D-lactic acid as shown in Figure 1.1. L-lactic acid is preferentially produced by bacterial fermentation. The content of D-lactic in the macromolecular chain and the molecular weight determine polymer properties such as the maximum achievable crystallinity [4]. PLA containing more than 93% of L-lactic is semi-crystalline contrary to PLA with L-lactic between 50 and 93% which is amorphous [1]. PLLA can reach 65% of crystallinity degree (Xc) [5] while a PLA with L-lactic content of 96% can crystallize only up to 40% Xc [6].

Crystalline morphology of PLA

PLA, like many semi-crystalline polymers, is polymorphic. It crystallizes under three main forms, dependent on the preparation conditions: ’ (or the newly termed “ ” [7], and . The and forms are obtained in specific conditions and therefore not found in packaging applications. The -polymorph is obtained by stretching PLLA fibers at high temperatures [8],[9]. The form is obtained with an epitaxial crystallization of PLLA chains [10]. PLLA and PDLA homopolymers can also crystallize under the form of a stereo-complex with distinct properties. It has triclinic geometry with lattice parameters equal to a = b = 0.916 nm and c = 0.87 nm [11]. The particularity of this phase is its high melting point (230 °C), attributed to the presence of hydrogen links in the lattice.
Based on a conformational analysis of the PLA, De Santis and Kovacs were the first to give a representation of the crystal conformation [12], which its geometry is orthorhombic with lattice parameters equal to a = 1.06 nm, b = 0.61 nm and c = 2.88 nm. Zhang et al. [13] showed the existence of a ’-phase (or phase) considered as a distorted -phase. This phase has a pseudo hexagonal geometry with lattice parameters equal to a = b = 0.62 nm and c = 2.88 nm
[14]. It is obtained during thermal crystallization at low temperatures, i.e. below 100°C and it converts into when its thermal crystallized above 120°C. In the calorimetric analysis of PLA this conversion causes of an exothermic peak just before a multiple melting endotherm [15–17]
[18]. Crystallization between 100 and 120°C gives rise to the coexistence of and ’ crystal
structures [13,14]. The WAXS patterns of the and ’ forms of PLA are very similar, with small differences in 2 values for the two strongest reflections ((110/200) and (203)) and the apparition of a characteristic peak of ’form at ≈ ° [18,19] (Figure 1.2).
Figure 1.2. (a) PLLA intensity profile of and ’ crystalline forms (Reproduced from Zhang et al.[13]) and (b) WAXS profile of PLLA samples crystallized at different temperatures. At 24,5° the apparition of a characteristic peak of ’ (Reproduced from Di Lorenzo et al.[18]).
Because of the different structures of PLLA ’ and crystals, it is considered that the mechanical, thermal properties and biodegradability of PLLA crystallized at different crystallization temperature are different [20]. As a consequence of the disordered structure, Cocca et al showed that the ’ crystal leads to a lower modulus and barrier properties and to higher elongation at break compared to crystal [19].
The values of enthalpy of 100% crystalline polylactide found in literature can vary from 82 to 203 J/g [21–25]. However, the most used value for enthalpy of 100% crystalline polylactide in literature is 93.1 J/g. Due to the polymorphic nature of PLA leading to a melt recrystallization process and to its complex stereo-chemistry, this value may reasonably be debated.

Amorphous phase dynamics of PLA

The amorphous phase, contrary to the crystalline phase, is a disordered phase, which is studied considering its dynamic. The amorphous phase of polymers, like all liquids which are glass-forming, undergoes a slow-down of its relaxation dynamics at temperatures close to the glass transition temperature. Many studies [26–28] link this behavior to the increase in the energy barrier that a structural unit must pass for relaxation.
The description of the cooperative movement of structural units in a polymer at the glass transition introduced by Adam and Gibbs [29] giving rise to the Cooperative Rearranging Regions (CRRs) concept. The hypothesis is that molecules do not relax independently which means that the movement of a molecule depends on its close neighbours and it is only possible if a given number of adjacent structural units are also moving. The glass-forming liquid can therefore be divided into independent sub-systems (CRRs). These structural units are the smallest amorphous domain where a conformational rearrangement may occur without causing structural change at its boundary. Each CRR has its own glass transition and relaxation time. The average size of CRR is the cooperativity length (ξ) and can be estimated from the heterogeneity of the relaxation times related to the distribution of the glass transitions around the conventional glass transition of the polymer. Donth proposed a calorimetric method for the determination of the CRR size and formulated the following equation [30]:
T is the mean temperature fluctuation related to the dynamic glass transition of one CRR, kB the Boltzmann constant, the polymer density, (1/ C p ) is equal to (1/ C p ) glass (1/ C p )liquid and C p is the heat capacity and T the dynamic glass transition.
The mean value of CRR size for an amorphous sample of PLA at dynamic glass transition temperature is 3 nm [6]. There are same causes referenced in literature inducing changes in CRR size. For example, when polymer undergoes structural constraints such as confinement [31,32] or when it is plasticized [33].
Many works using the CRR concept to describe the amorphous phase dynamics in semi-crystalline polymers were published [6,34–36]. Schick and Donth [37] demonstrated that the CRR length of poly(ethylene terephthalate) (PET) is proportional to the thickness of self-organized the amorphous phase layers and it decreases when the crystalline fraction increases. Codou et al. [38] found the same behavior for poly(ethylene 2,5-furandicarboxylate) (PEF) and PET. The increase of the crystallinity degree decreased the CRR length (Figure 1.3). Other authors have also seen the same behavior, as Lixon et al. [35] for PET, Delpouve et al. [36,39] and Saiter et al. [34] for PLLA. Furushima et al. [40] studied the characteristic lenght of CRR for uniaxial drawn PET films and they observed that CRR size increases linearly with increasing MAF. However, a non linear behavior was observed between crystallinity and CRR size.
Delpouve et al. [36] studied the cooperativity length in drawn PLA. The macromolecular orientation generated by the drawing process above Tg led to the generation of oriented crystals and geometric restrictions in the amorphous phase. A decrease in the cooperativity length with increasing degree of crystallinity was observed. This was attributed to the geometric restrictions of the amorphous fraction between crystallites, which induced a geometrical confinement of the macromolecular chains (Figure 1.4).

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Formation of the Rigid Amorphous Fraction in semicrystalline polymers

In semicrystalline polymers, such as PET or PLA, a three-phase model, introduced by Wunderlich et al. [41], is used to describe the organization of the material at the nanometric scale. This concept was introduced to explain the deviation between crystallinity and heat capacity in the glass transition for several polymers [42]. In the three phase model (sketched in Figure 1.5), next to the crystalline phase, two fractions can be distinguished in the amorphous phase. The mobile amorphous fraction (MAF), which relaxes at the glass transition and the rigid fraction (RAF), which devitrifies a higher temperature. The RAF is the intermediate phase connecting the crystalline phase to the mobile amorphous fraction (MAF) [43] as we can see in the scheme in Figure 1.5. It constitutes moreover a dedensified amorphous phase [44] because of the geometrical constraints preventing structure relaxation. Because it requires more energy for relaxation, the RAF does not participate of the glass transition. Wunderlich [45] showed that the RAF does not stay rigid up to the melting, it disappears progressively above Tg. Depending on the polymer and the conditions of crystallization, the RAF can devitrify in a very wide temperature range, even above the melting temperature of the crystals [46].

Table of contents :

1. Literature Review
1.1. Characterization of PLA
1.1.1. Synthesis of PLA
1.1.2. Crystalline morphology of PLA
1.1.3. Amorphous phase dynamics of PLA
1.2. Formation of the Rigid Amorphous Fraction in semicrystalline polymers
1.2.1. Properties of the Rigid Amorphous Fraction in PET
1.2.2. Properties of the Rigid Amorphous Fraction in PLA
1.3. Gas barrier properties of PLA
1.4. Confinement of polymers
1.4.1. Different methods of confinement of polymers
1.4.2. Crystallization of polymers under confinement
1.4.3. Amorphous phase relaxation in confined polymers
1.4.4. Gas barrier properties of confined polymers using the nanolayer-multiplying system
1.5. Conclusion
1.6. References
2. Matériaux et méthodes
2.1. Matériaux
2.1.1. Choix des polymères
2.2. Méthodes expérimentales
2.2.1. Caractérisation des granulés Séchage des granulés de PLA, et PC Description des sécheurs à granulés Protocole expérimental de séchage de granulés
2.2.2. Rhéologie
2.2.3. Mise en oeuvre des films Mise en oeuvre des films monocouche Mise en oeuvre de films multicouches (PLLA/PS et PLLA/PC)
2.2.4. Caractérisation des films Perméabilité à l’oxygène Perméabilité à l’hélium Analyse thermique par MT-DSC Protocoles expérimentaux Quantification des phases Longueur de coopérativité Diffractions des rayons x Recuit des films monocouche Cinétique de cristallisation Recuit des films multicouches Recuit des films PS/PLLA . Cinétique de cristallisation des films PS/PLLA Recuit des films PC/PLLA Cinétique de cristallisation des films PC/PLLA Observation des structures cristallines Digestion enzymatique et Microscopie électronique à balayage Observation des couches Analyse des images AFM Mesure des épaisseurs des couches et vérification de la proportion de chaque polymère dans les films Caractérisation du vieillissement physique des films multicouches PLLA/PS
2.3. Bibliographie
3. Multi-scale analysis of the impact of polylactide morphology on gas barrier properties
3.1. Abstract
3.2. Introduction
3.3. Material and Methods
3.3.1. Sample preparation
3.3.2. Sample characterization
3.4. Results and Discussion
3.4.1. PLA crystallization kinetics
3.4.2. Crystalline structure and space filling
3.4.3. Crystalline morphology
3.4.4. Structural description of the amorphous phase and its cooperativity at the glass transition
3.4.5. Oxygen permeability study
3.5. Conclusion
3.6. References
4. Structural and dynamic heterogeneity in the amorphous phase of PLLA confined at the nanoscale by co-extrusion process
4.1. Abstract
4.2. Introduction
4.3. Experimental
4.3.1. Process and crystallization parameters
4.3.2. Characterization
4.4. Results and discussion
4.4.1. Optimization of the layer multiplying co-extrusion process
4.4.2. Crystallization of PLLA under confinement
4.4.3. Dynamic heterogeneity of confined PLLA
4.5. Conclusion
4.6. References
4.7. Supplementary information
5. The impact of nanoconfinement on PLLA crystallization and gas barrier properties
5.1. Abstract
5.2. Introduction
5.3. Materials and methods
5.3.1. Materials and processing
5.3.2. Methods
5.4. Results and discussion
5.4.1. Optimization of the layer multiplying co-extrusion process
5.4.2. Crystallization kinetics and crystalline lamellae orientation of confined PLLA layers
5.4.3. Impact of the layer multiplying co-extrusion on the confining polymer PC ….
5.4.4. Maximum crystallinity degree and amorphous phase dynamics and of confined PLLA
5.4.5. Impact of PLLA microstructure on helium and oxygen permeability
5.5. Conclusion
5.6. References
6. Physical aging in PLLA confined at the nanoscale by multilayer coextrusion
6.1. Introduction
6.2. Materials and methods
6.2.1. Characterization of physical aging
6.3. Results and discussion
6.4. Conclusion
6.5. References


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