**1. Introduction**

140 Recent Advances in Plasticizers

Lenik, J.; Wardak, C. & Marczewska, B. (2008). Propreties of Naxopen Ion-Selective Electrodes. *Central European Journal of Chemistr,* Vol. 6, No. 4, pp. 513-519 Lizondo-Sabater, J.; Martinez-Manez, R.; Sancenon, F.; Segui, J. & Soto, J. (2008). Ion-

Masadome, T.; Yang, J-G. & Toshihiko, I. (2004). Effect of Plasticizer on the Performance of

Matesic-Puac,. R., Sak-Bosnarb, M., Bilica, M. & Bozidar S. Grabaric, B. (2005).

Mihali, C. (2006), Researches Regarding Preparation of Electrochemical Sensors for Anionic

Mihali, C.; Oprea, G. & Cical, E. (2008). Determination of Critical Micelar Concentration of

Mihali, C.; Oprea, G. & Cical, E. (2009). PVC matrix ionic – surfactant selective electrodes

Moody, G. & Thomas J. (1986). Progress in designing calcium ion-selective electrodes. *Ion* 

Nazarov, V. A.; Sokolova, E. I.; Androchik, K. A.; Egorov, V. V.; Belyaev, S. A. &

Oesch, U., Ammann, D. & Simon, W. (1886). Ion-Selective Membrane Electrodes for Clinical

Oprea, G., Mihali. C. & Hopirtean, E. (2007). Anionic Surfactants Selective Electrodes Based

O'Rourke, M.; Duffy, N.; De Marco, R. & Potter, I. (2011). Electrochemical Impedance

Trebbe, U.; Niggermann, M.; Cammann, K.; Fiaccabrino, G. C.; Kundelka-Hep, M.;

no Added Ion-Exchanger. *Mirochimia Acta,* No. 144, pp. 217-220

*"POLITEHNICA" Univ. (Timisoara),* Vol. 53(67), pp. 159-162

Babes-Bolyai – Chemia series, Vol. LIV, No. 3, pp. 141-150

Ionophore. *Talanta*, Vol. 75, pp. 317-325

Surfactants. *Ph. D. Thesis,* pp. 5-11

*Selective Electrode Rev.,* Vol. 1, pp. 3-30

No. 3, pp. 335-338, ISSN 0034-7752

Vol. 65, No. 9, pp. 960-963, ISSN 1061-9348

Use. *Clin. Chem.* Vol. 32, No. 8, pp. 1448-1459

doi:10.3390/membranes102013, ISSN 2077-0375

*Analytical Chemistry*, Vol. 371, pp.734-739

228.

Selective Electrodes for Anionic Surfactants Using a Cyclam Derivative as

the Surfactant-Selective Electrode Based on a Poly(Vinyl Chloride) Membrane with

Potentiometric determination of anionic surfactants using a new ion-pair-based allsolid state surfactant sensitive electrode. *Sensors and Actuators B*, Vol 106 pp. 221–

Anionic Surfactants Using Surfactants - Sensible Electrodes. *Chem. Bull.* 

based on the ionic pair tetraalkyl-ammonium-laurylsulphate, Studia Universitatis

Yurkshtovich, T. L. (2010). Ibuprofen – Selective Electrode on the Basis of a Neutral Carrier, N-Trifluoroacetylbenzoic Acid Heptyl Ester. *Journal of Analytical Chemistry*,

On Tricaprylmethyl Ammonium Laurylsulphate Ionophore. *Rev. Chim*., Vol. 58,

Spectroscopy—A Simple Method for the Characterization of Polymer Inclusion Membranes Containing Aliquat 336. *Membranes* 1, pp. 132-148;

Dzyadevich, S. & Shulga, O. (2001). A New Calcium Sensor Based on Ion-Selective Conductometric Microsensor-Membranes and Features. *Fresenius Journal of*  Organic/inorganic polymer hybrids is a rapidly growing area of research because they offer opportunities to combine desirable properties of organic polymers (toughness, elasticity, formability) with those of inorganic solids (hardness, chemical resistance, strength). There are several routes to prepare hybrid materials, but one of the most common method is solgel technique generating inorganic phase within organic polymer matrix. The advantage of sol–gel technique is mild processing characteristics and the possibility of tailoring morphology of the growing inorganic phase and thus properties of the material by the subtle control of various reaction conditions. This process includes hydrolysis of the precursor (metal alkoxide) followed by condensation reactions of the resulting hydroxyl groups. Considering the nature of the interface between the organic and inorganic phases, hybrid materials can be categorized into two different classes. The first class corresponds to non-covalently bound networks of inorganic and organic phases. These hybrids show weak interactions between the polymer matrix and inorganic phase, such as van der Waals, hydrogen bonding or weak electrostatic interactions and can be prepared by physical mixing of an organic polymer with a metal alkoxide. In the second class organic and inorganic phases are linked through strong chemical bonds (covalent or ionic). Chemical bonding can be achieved by the incorporation of silane coupling groups into organic polymers [1-3].

Cellulose has received a great deal of attention in recent decades as a substitute for petrochemical based polymers. Natural polymer shows however some limitations, for instance with regard to poor processability or high water absorbency. Cellulose esters such as cellulose acetate (CA), cellulose acetate propionate (CAP) and cellulose acetate butyrate (CAB) are less hydrophilic than cellulose, thermoplastic materials [4]. To improve their processability and mechanical properties, the addition of plasticizers is usable. Plasticizers as polymer additives serve to decrease the intermolecular forces between the polymer chains, resulting in a softened and flexible polymeric matrix. They increase the polymer's elongation and enhance processability by lowering the melting and softening points and viscosity of the melts [5].

The Effect of Concentration and Type of Plasticizer

**3. Mechanisms of plasticization** 

polymer matrix.

gel flexibility.

the glass transition temperature lowers.

polymer [7].

on the Mechanical Properties of Cellulose Acetate Butyrate Organic-Inorganic Hybrids 143

be used as lower cost, partial replacement for a primary plasticizer. It is possible that a plasticizer used in one formulation as a primary plasticizer could be used in a second formulation as a second one [10, 11]. Plasticizers, especially used in biopolymer-based films, can also be classified as water soluble and water insoluble. Hydrophilic plasticizers dissolve in polymeric aqueous dispersions and may cause an increase of water diffusion in the polymer when added in high concentration. On the contrary, hydrophobic plasticizers can lead to a decrease in water uptake, due to the closing of micro-voids in the

There are several theories that describe the effects of plasticizers and a combination of them

a. Lubricity theory, developed by Kilpatrick, Clark and Houwink, among others, states that plasticizer acts as a lubricant, reducing intermolecular friction between polymer molecules responsible for rigidity of the polymer. On heating, the plasticizer molecules slip between polymer chains and weaken the polymer-polymer interactions (van der Waals' forces), shielding polymer chains from each other. This prevents the reformation of a rigid network, resulting in more flexible, softener and distensible

b. Gel theory, developed by Aiken and others, holds that polymers are formed by an internal three-dimensional network of weak secondary bonding forces (van der Waals' forces, hydrogen bonding) sustained by loose attachments between the polymer molecules along their chains. These bounding forces, are easily overcome by external strain applied to the material, allowing the plasticized polymer to be bend, stretch, or compress. Plasticizer molecules attach along the polymer chains, reducing the number of the polymer-polymer attachments and hindering the forces holding polymer chains together. The plasticizer by its presence separates the polymer chains and increases the space between polymer molecules, thus reducing the rigidity of the gel structure. Moreover, plasticizer molecules that are not attached to polymer tend to aggregate allowing the polymer molecules to move more freely, thus enhancing the

c. Free volume theory holds that the presence of a plasticizer lowers the glass transition temperature (Tg) of the polymer. Free volume is a measure of internal space available within a polymer matrix. There are three main sources of free volume in polymer: motion of polymer end groups, motion of polymer side groups, and internal polymer motions. When the free volume increases, more space or free volume is provided for molecular or polymer chain movement. A polymer in the glassy state has an internal structure with molecules packed closely and small free volume. This makes the material rigid and hard. When the polymer is heated to above the glass transition temperature, the thermal energy and molecular vibrations create additional free volume which allows greater internal chain rotation and an increase in the segment mobility. This makes the system more flexible and rubbery. When small molecules such as plasticizers are added, the free volume available to polymer chain segments increases and therefore

allows to explain the concept of polymer plasticization [8, 10, 13-15]:

Plasticizers are often inert organic compounds with low molecular weight, high boiling points and low vapor pressures that are used as polymer additives. The main role of the plasticizer is to improve mechanical properties of the polymers by increasing flexibility, decreasing tensile strength and lowering the second order transition temperature [6]. The International Union of Pure and Applied Chemistry (IUPAC) developed a definition for a plasticizer as a "substance or material incorporated in a material (usually a plastic or an elastomer) to increase its flexibility, workability, or distensibility" [7]. Attributes of a good plasticizer are good compatibility with polymer, which depends on polarity, solubility, structural configuration and molecular weight of plasticizer and results from a similar chemical structure of polymer and plasticizer. Other important factor is plasticizer permanence related to its resistance to migration. Therefore, a good plasticizer should have high boiling point and low volatility (low vapor pressure) to prevent or reduce its loss during processing. Plasticizers should also be aroma free and non-toxic. Another important feature is low rate of migration out of material to preserve desirable properties of plasticized polymer and avoid contamination of the materials from the point of potential health and environmental impacts in contact with it. The permanence of plasticizer in polymer is dependent on the size of the plasticizer molecule, thus the larger molecules, the greater permanence of the plasticizer. The higher diffusion rate of plasticizer in the polymer, the lower permanence due to the migration out of the polymer matrix [8, 9]. Plasticizers influence also processing of the polymers by changing various parameters: viscosity, filler incorporation, dispersion rate, flow, power demand and heat generation [7]. A good plasticizer should also be insensitive to solar UV radiation, stable in a wide temperature range and inexpensive [6]. The efficiency of a plasticizer is defined as the quantity of plasticizer required to provide desired mechanical properties of obtained material [8]. Taking into consideration that effective plasticization is depended on such factors as: chemical structure of the plasticizer, its compatibility and miscibility with the polymer, molecular weight and concentration of plasticizer, rate of diffusion of the plasticizer into the polymer matrix, different polymers require different plasticizers [8].

#### **2. Plasticizer classification**

There are two techniques for plasticization: external and internal. External plasticization is a method that provides plasticity through physical mixing. Thus, external plasticizers are not chemically bound to the polymer and can evaporate, migrate or exude from polymer products by liquid extraction [6]. Plasticization of polymers by incorporation of comonomers or reaction with the polymer, providing flexible chain units is called an internal plasticization. Internal plasticizers are groups (flexible segments) constituting a part of a basic polymer chain, which may be incorporated regularly or irregularly between inflexible monomers (hard segments) or grafted as side chains thus reducing intermolecular forces [7, 10-12]. According to the compatibility with the polymer, external plasticizers can be classified into two principal groups: primary and secondary ones, called also extenders. Primary plasticizers have a sufficient level of compatibility with polymer to be able to be used as sole plasticizer in all reasonable proportions, giving a desirable modifying effect. They interact directly with chains. Secondary plasticizers have limited compatibility and will exude from the polymer if used alone. They are used along with the primary plasticizer, as a part of plasticizer system, to meet a secondary performance requirements (cost, low-temperature properties, permanence). Extenders can be used as lower cost, partial replacement for a primary plasticizer. It is possible that a plasticizer used in one formulation as a primary plasticizer could be used in a second formulation as a second one [10, 11]. Plasticizers, especially used in biopolymer-based films, can also be classified as water soluble and water insoluble. Hydrophilic plasticizers dissolve in polymeric aqueous dispersions and may cause an increase of water diffusion in the polymer when added in high concentration. On the contrary, hydrophobic plasticizers can lead to a decrease in water uptake, due to the closing of micro-voids in the polymer [7].
