**Control of polymer structure in processing conditions**

Industrial polymer activities, through processes like, for instance, extrusion coating (*i.e.*, the food industry with consumption products), injection moulding (*i.e.*, the industry with

Thermodynamics and Thermokinetics to Model Phase Transitions of Polymers

**2.2 Development of combined experimental procedures** 

polymers under various, coupled and extreme conditions.

**2.2.1 Temperature control at atmospheric pressure** 

nanocalorimetry (Schick, 2009) are the main designs.

**2.2.2 Temperature-pressure-volume control** 

The major difficulty is to generate high pressure.

are high cooling rate control and shearing rate.

over Extended Temperature and Pressure Ranges Under Various Hydrostatic Fluids 645

The coupling of thermodynamic and kinetic effects (*i.e.*, confinement, shear flow, thermal gradient) with diffusion (*i.e.*, pressurizing sorption,) and chemical environment *(i.e.*, polar effect, oxidation), and the consideration of the nature of the polymers *(i.e.*, homopolymers, copolymers, etc.) require a broad range of indispensable *in-situ* investigations. They aim at providing well-documented thermodynamic properties and phase transitions profiles of

Usual developed devices are based on the control of temperature, while the main concerns

The kinetic data of polymer crystallization are often determined in isothermal conditions or at moderate cooling rates. The expressions are frequently interpreted using simplified forms of Avrami's theory involving thus Avrami's exponent and a temperature function, which can be derived from Hoffman-Lauritzen's equation **(**Devisme et al., 2007). However, such an interpretation cannot be extrapolated to low crystallization temperatures encountered in polymer processing, *i.e.*, to high cooling rates (Magill, 1961, 1962, 2001; Haudin et al., 2008; Boyer et al., 2011b). In front of the necessity for obtaining crystallization data at high cooling rates, different technical solutions are proposed. Specific hot stages (Ding & Spruiell, 1996; Boyer & Haudin, 2010), quenching of thin polymer films (Brucato et al., 2002), and

Similarly, to generate a controlled melt shearing, various shearing devices have been proposed, for instance, home-made sliding plate (Haudin et al., 2008) and rotating parallel plate devices (*e.g.*, Linkam temperature controlled stage, Haake Mars modular advanced rheometer system). The shear-induced crystallization can be performed according to a 'long' shearing protocol as compared to the 'short-term' shearing protocol proposed by the group

The design of devices based on the control of pressure requires breakthrough technologies.

In polymer solidification, the effects of pressure can be studied through pressure–volume– temperature phase diagrams obtained during cooling at constant pressure. The effect of hydrostatic (or inert) pressure on phase transitions is to shift the equilibrium temperature to higher values, *e.g.*, the isotropic phase changes of complex compounds as illustrated in the works of Maeda et al. (2005) by high-pressure differential thermal analyzer and of Boyer et al. (2006a) by high-pressure scanning transitiometry, or the melting temperature in polymer crystallization as illustrated for polypropylene in the work of Fulchiron et al. (2001) by highpressure dilatometry. However, classical dilatometers cannot be operated at high cooling rate without preventing the occurrence of a thermal gradient within the sample. This problem can be solved by modelling the dilatometry experiment (Fulchiron et al., 2001) or by using a miniaturized dilatometer (Van der Beek et al., 2005). Alternatively, other promising technological developments propose to couple the pressure and cooling rates as shown with an apparatus for solidification based on the confining fluid technique as described by Sorrentino et al. (2005). The coupling of pressure and shear is possible with the shear flow pressure–volume–temperature measurement system developed by Watanabe et

of Janeschitz-Kriegl (Janeschitz-Kriegl et al., 2003, 2006; Baert et al., 2006).

engineering parts for automotive or medicine needs) (Devisme et al., 2007; Haudin et al., 2008), deal with polymer formulation and transformation. The viscous polymer melt partially crystallizes after undergoing a complex flow history or during flow, under temperature gradients and imposed pressure (Watanabe et al., 2003; Elmoumni & Winter, 2006) resulting into a non homogeneous final macrometric structure throughout the thickness of the processed part. The final morphologies are various sizes and shapes of more or less deformed spherulites resulting from several origins: *i)* isotropic spherulites by static crystallization (Ferreiro et al., 2002a; Nowacki et al., 2004), *ii)* highly anisotropic morphologies as oriented and row-nucleated structures (*i.e.*, shish-kebabs) by specific shear stress (Janeschitz-Kriegl, 2006; Ogino et al., 2006), *iii)* transcrystalline layer (as columnar pattern in metallurgy) by surface nucleation and/or temperature gradient, and *iv)* teardrop- -shaped spherulites or "comets" (spherulites with a quasi-parabolic outline) by temperature gradients (Ratajski & Janeschitz-Kriegl, 1996; Pawlak et al., 2002).

Together with the deformation path (*e.g.*, tension, compression), the morphology strongly influences the behaviour of polymers. Some models have attempted to predict the properties of spherulites through a simulation of random distributions of flat ellipsoids (crystalline lamellae) embedded in an amorphous phase described by a finite extensible rubber network (Ahzi et al., 1991; Dahoun et al., 1991; Arruda & Boyce, 1993; Bedoui et al., 2006).

Moreover by considering the high-pressure technology, the use of specific fluids plays a non negligible role in pattern control. The thermodynamic phase diagrams of fluids implies the three coordinates (pressure-volume-temperature, *PVT*, variables) representation where the fluids can be in the solid, gaseous, liquid and even supercritical state. The so-called "signature of life" water (H2O) (Glasser, 2004) and the so-called "green solvent" in fact "clean safe" carbon dioxide (CO2) (Glasser, 2002) can be cited. The use of H2O is encountered in injection moulding assisted with water. CO2 is known as a valuable agent in polymer processing thanks to its aptitude to solubilize, to plasticize (Boyer & Grolier, 2005), to reduce viscosity, to favour polymer blending or to polymerize (Varma-Nair et al., 2003; Nalawade et al., 2006). In polymer foaming, elevated temperatures and pressures are involved as well as the addition of chemicals, mostly penetrating agents that act as blowing agents (Tomasko et al., 2003; Lee et al., 2005).

### **Damage of polymer structure in on-duty conditions**

In the transport of fluids, in particular in the petroleum industry taken as an example, flexible hosepipes are used which engineering structures contain extruded thermoplastic or rubber sheaths together with reinforcing metallic armour layers. Transported fluids contain important amounts of dissolved species, which on operating temperature and pressure may influence the resistance of the engineering structures depending on the thermodynamic *T*, *P*-conditions and various phenomena as sorption/diffusion, chemical interactions (reactive fluids, *i.e.*, oxidation), mechanical (confinement) changes. The polymer damage occurs when rupture of the thermodynamic equilibrium (*i.e.*, after a sharp pressure drop) activates the blistering phenomenon, usually termed as 'explosive decompression failure' (XDF) process (Dewimille et al., 1993; Rambert et al., 2006; Boyer et al., 2007; Baudet et al., 2009). Damage is a direct result of specific interactions between semi-crystalline patterns and solvent with a preferential interaction (but not exclusive) in the amorphous phase (Klopffer & Flaconnèche, 2001).
