Melting temperature copolymers

Natta, Porri, Carbonaro and Lugli (25) have prepared copolymers of 1,3-butadiene with 1,3-pentadiene in the whole range of compositions. The properties of the copolymers, in which all butadiene and pentadiene comonomer units are in the trans-1,4 configuration, clearly show the isomorphous replacement between the two types of units. The melting point/composition data show that the copolymer melting temperatures are a regular function of composition and are always comprised between those of trans-1,4-polybutadiene modification II and trans-1,4-polypentadiene. Also the X-ray diffraction spectra of the copolymers show that the trans-1,4-pentadiene units are isomorphous with the trans-1,4-butadiene units crystallized in the crystalline modification of the latter stable at high temperatures (form II). 

Figure 11.1, the experimentally determined copolymer melting temperature decreases with increasing counit content, in agreement with Flory s model however, the numerical values of the measured melting temperatures are significantly below those calculated from Equation 11.2, and the dependence of on counit content is much stronger. 

Statistical copolymers of ethylene terephthalate (ET) and 1,4-cyclohexylene dimethylene terephthalate (CT) were also found to co-crystallize over a limited composition range [84, 85]. The copolymers rich in ET were found to form a crystalline phase that contained only ET units, while in the copolymers rich in CT, the crystalline phase contained both ET and CT units. The transition between these two crystal phases, as a function of composition, corresponded to a minimum in both copolymer melting temperature and heat of crystallization. It was speculated that the transition occurs at a composition where the energy difference between the melt and the crystal is identical for the two comonomers [85]. 

Thus far, we have discussed a number of key experimental observations regarding the effects of counit incorporation on the solid-state structure and the crystallization kinetics in statistical copolymers. In order to better quantify these experimental observations, various thermodynamic models have been proposed. Rory s model, as outlined in Section 11.2.1, correctly describes the equilibrium melting behavior of copolymers in the limit of complete comonomer exclusion however, it is often found to be inadequate at predicting experimentally accessible copolymer melting temperatures [11-14]. An alternative was proposed by Baur [91], where each polymer sequence is treated as a separate molecule with an average sequence length in the melt given by [91] ... 

Figure 11.9 Poly(ET-c6>-EN) statistical copolymer melting point and unit-cell data, shown as a function of composition (a) copolymer melting temperature, (b) projection of the a unit cell length onto the plane normal to the chain axis, a (c) projection of the b unit cell length onto the plane normal to the chain axis, b(d) c unit-cell length. In panels (b-d), filled symbols indicate a PEN-like crystal structure, while open symbols correspond to a PET-like structure. Reprinted from Reference [86] with permission of Elsevier, Copyright 1995.

Sanchez and Eby analyzed the case where an arbitrary amount of the counits, Xcb, is included in the copolymer crystalline phase [100]. Changes in the system s bulk free energy were considered, and from that equations for determining copolymer melting temperature were proposed. For crystals of infinite thickness and composition Xcb, the copolymer melting temperature is given by [100] ... 

Sanchez and Eby also considered the case where the crystals have a finite thickness, /, and fold surface free energy, Oe [100]. Then in place of Equation (11.11), the copolymer melting temperature can be calculated using the following expression ... 

The Sanchez-Eby and Wendling-Suter models, which differ mainly in the limit of large e, have been successfully applied to describe experimental copolymer crystallization data, especially when counit inclusion in the crystal unit cell is substantial (i.e., e is small) [88,101]. In one such example, Marchessault and colleagues studied the isodimorphic crystallization behavior in poly(j8-hydroxybutyrate-C(9-/3-hydroxyvalerate) statistical copolymers [87, 88]. The experimentally measured copolymer melting temperatures were found to be well described by Sanchez and Eby s model with the assumptions that the crystals were of finite thickness (i.e., Eq. 11.12) and defect inclusion was uniform (i.e., Xcb = b) [88]. 

Figure 11.11 Poly(ET-c6>-EN) statistical copolymer melting temperature (T ) as a function of EN unit mole fraction Xen)-The experimental data (solid circles) are compared with different theoretical models (Flory model, gray solid line Baur model, dotted line Wendling-Suter model, black solid line, calculated with values of e/RT indicated). Reprinted with permission from Reference [101]. Copyright 1998 American Chemical Society.

This type of adhesive is generally useful in the temperature range where the material is either leathery or mbbery, ie, between the glass-transition temperature and the melt temperature. Hot-melt adhesives are based on thermoplastic polymers that may be compounded or uncompounded ethylene—vinyl acetate copolymers, paraffin waxes, polypropylene, phenoxy resins, styrene—butadiene copolymers, ethylene—ethyl acrylate copolymers, and low, and low density polypropylene are used in the compounded state polyesters, polyamides, and polyurethanes are used in the mosdy uncompounded state. 

The properties of PVDC (Table 3) ate usually modified by copolymerization. Copolymers of high VDC content have lower melting temperatures than PVDC. Copolymers containing mote than mol % acrylate or methacrylate ate amorphous. Substantially mote acrylonitrile (25%) or vinyl chloride (45%) is required to destroy crystallinity completely. 

The effect of different types of comonomers on varies. VDC—MA copolymers mote closely obey Flory s melting-point depression theory than do copolymers with VC or AN. Studies have shown that, for the copolymers of VDC with MA, Flory s theory needs modification to include both lamella thickness and surface free energy (69). The VDC—VC and VDC—AN copolymers typically have severe composition drift, therefore most of the comonomer units do not belong to crystallizing chains. Hence, they neither enter the crystal as defects nor cause lamellar thickness to decrease, so the depression of the melting temperature is less than expected. 

Fig. 3. Influence of vinyl alcohol—vinyl acetate copolymer composition on melting temperature (56), where A represents block copolymers B, blocky...

Vinyhdene chloride copolymers are available as resins for extmsion, latices for coating, and resins for solvent coating. Comonomer levels range from 5 to 20 wt %. Common comonomers are vinyl chloride, acrylonitrile, and alkyl acrylates. The permeability of the polymer is a function of type and amount of comonomer. As the comonomer fraction of these semicrystalline copolymers is increased, the melting temperature decreases and the permeability increases. The permeability of vinylidene chloride homopolymer has not been measured. 

Multiblock Copolymers. Replacement of conventional vulcanized mbber is the main appHcation for the polar polyurethane, polyester, and polyamide block copolymers. Like styrenic block copolymers, they can be molded or extmded using equipment designed for processing thermoplastics. Melt temperatures during processing are between 175 and 225°C, and predrying is requited scrap is reusable. They are mostiy used as essentially pure materials, although some work on blends with various thermoplastics such as plasticized and unplasticized PVC and also ABS and polycarbonate (14,18,67—69) has been reported. Plasticizers intended for use with PVC have also been blended with polyester block copolymers (67). 

C and is easily processable, whereas the homopolymers do not melt before the onset of thermal degradation, at temperatures as high as 500°C.73,74 Varying copolymer composition permits the adjustment of melting temperature and of other properties (e.g., solubility) to desired values. This method is frequently used for aliphatic and aromatic-aliphatic polyesters as well. 

Another important type of condensation polymer are the linear polyesters, such as poly (ethylene terephthalate) (PET) and poly (butylene terephthalate) (PBT). Copolymers of polyesters and PA have been studied in detail, and it has been shown that random copolyesteramides have a low structural order and a low melting temperature. This is even the case for structurally similar systems such as when the group between the ester unit is the same as that between the amide unit, as in caprolactam-caprolactone copolymers (Fig. 3.10).22 Esters and amide units have different cell structures and the structures are not therefore isomorphous. If block copolymers are formed of ester and amide segments, then two melting temperatures are present. 

Figure 3.10 Melting temperatures of caprolactam-caprolactone random copolymers as function of molar composition.22...

PAs have also been copolymerized with other polymer systems and, in particular", with polyesters and poly ethers. In the copoly esteramides the crystallinity is decreased by copolymerization, as the crystalline structure of the amide unit is very different from the ester unit. However, alternating polyesteramides behave as homopolymers with a glass ttansition temperature and a melting temperature intermediate to the polyester and the PA polymer (Figs. 3.10 and 3.11).23,24 Polyesters, such as PBT and PET, modified with a small amount of diamide are also copolymers that have a high order.24,73... 

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