**4. Materials and methods**

Cellulose acetate butyrate (CAB, Mn≈ 70000), TEOS (98%) and TEA (99%) were purchased from Sigma-Aldrich. TEC (98%) and TBC (97%) were purchased from Fluka. DEP (99%), DBP (99%) and DOP (99%) were purchased from POCH and used as received. Organicinorganic hybrids were synthesized according to the procedure we described in patent number 209829 [35]. Cellulose acetate butyrate hybrids were prepared with various amounts of TEOS: 6,25 wt.% and 12,5 wt.%, and various amounts of the chosen plasticizer (25-35%), such as biodegradable citrates: TEC, TEA, TBC and conventional phthalates: DEP, DBP, DOP. Obtained films showed thickness in the range of 0,15-0,18 mm. Samples prepared with concentration below 25% of all investigated plasticizers were too brittle for tensile testing.

Sample compositions and codes are as follows:


A typical preparation of organic-inorganic hybrid was as follows [36]: polymer was placed in a polyethylene beaker and dissolved in acetone. Plasticizer and TEOS was then added and mixed vigorously. To this solution catalytic amount of HCl (0.1 M) was added to initiate the sol-gel process and mixed until it appeared clear and homogenous. The solution was cast in an evaporating PTFE dish and left exposed to atmospheric conditions followed by drying in a vacuum drier at 40oC for 12 hours to ensure complete solvent evaporation.

Mechanical properties were investigated using a universal tensile machine (Instron 5565) at a crosshead speed of 100 mm/min at room temperature (according to the test method described in International Standards PN-EN ISO 527-1:1998, PN-EN ISO-3:1998). Sample dimensions: length 150 mm, width 10 mm. At least five tests were performed for each type of the sample, to ensure the reliability of the test results, and the average was used.

The properties of the materials used in this study are showed in Table 1.

In our previous work we examined the effect of inorganic phase amount and diethyl phthalate and citrate plasticizer on the degradability of organic-inorganic cellulose acetate butyrate films in sea water [34]. The results of our study showed that the higher the amount of silica incorporated into the CAB with the DEP plasticizer, the higher degradability of the samples. The experiment also showed a synergistic effect of the applied plasticizer on the degradation rate of the CAB/silica hybrids. The CAB/silica hybrids with diethyl phthalate were degraded faster than the hybrids with tributyl citrate due to the higher brittleness of those samples. The aim of the present study is to examine the effects of six different plasticizers: citrate esters and phthalates, on the mechanical properties of cellulose acetate

Cellulose acetate butyrate (CAB, Mn≈ 70000), TEOS (98%) and TEA (99%) were purchased from Sigma-Aldrich. TEC (98%) and TBC (97%) were purchased from Fluka. DEP (99%), DBP (99%) and DOP (99%) were purchased from POCH and used as received. Organicinorganic hybrids were synthesized according to the procedure we described in patent number 209829 [35]. Cellulose acetate butyrate hybrids were prepared with various amounts of TEOS: 6,25 wt.% and 12,5 wt.%, and various amounts of the chosen plasticizer (25-35%), such as biodegradable citrates: TEC, TEA, TBC and conventional phthalates: DEP, DBP, DOP. Obtained films showed thickness in the range of 0,15-0,18 mm. Samples prepared with concentration below 25% of all investigated plasticizers were too brittle for

a. samples prepared from plasticized CAB: short name of plasticizer (TEC, TEA, TBC,

b. organic-inorganic hybrids prepared from composition of plasticized CAB and TEOS in 87.5/12.5 polymer/TEOS ratio: amount of TEOS/short name of plasticizer (TEC, TEA,

c. organic-inorganic hybrids prepared from composition of plasticized CAB and TEOS in 93.75/6.25 polymer/TEOS ratio: amount of TEOS/short name of plasticizer (TEC, TEA,

A typical preparation of organic-inorganic hybrid was as follows [36]: polymer was placed in a polyethylene beaker and dissolved in acetone. Plasticizer and TEOS was then added and mixed vigorously. To this solution catalytic amount of HCl (0.1 M) was added to initiate the sol-gel process and mixed until it appeared clear and homogenous. The solution was cast in an evaporating PTFE dish and left exposed to atmospheric conditions followed by drying in a vacuum drier at 40oC for 12 hours to ensure complete solvent evaporation.

Mechanical properties were investigated using a universal tensile machine (Instron 5565) at a crosshead speed of 100 mm/min at room temperature (according to the test method described in International Standards PN-EN ISO 527-1:1998, PN-EN ISO-3:1998). Sample dimensions: length 150 mm, width 10 mm. At least five tests were performed for each type

of the sample, to ensure the reliability of the test results, and the average was used.

The properties of the materials used in this study are showed in Table 1.

TBC, DEP, DBP or DOP)/plasticizer content, e.g. 12.5TEC25, 12.5DBP30,

TBC, DEP, DBP or DOP)/plasticizer content, e.g. 6.25TEC25, 6.25DBP30.

butyrate hybrids.

tensile testing.

**4. Materials and methods** 

Sample compositions and codes are as follows:

DEP, DBP or DOP)/plasticizer content, e.g. TEC25, DOP35,



The Effect of Concentration and Type of Plasticizer

CAB hybrids was insufficient for providing acceptable flexibility.

on the Mechanical Properties of Cellulose Acetate Butyrate Organic-Inorganic Hybrids 149

The aim of adding plasticizer to CAB-hybrids is to reduce natural brittleness of the polymer and to enhance plastic elongation, while providing optimal tensile strength and stiffness.

The plasticizing efficiency of the investigated phthalates and citrates evaluated by tensile testing is summarized in Table 2. At concentration 25% samples of the cellulose acetate butyrate plasticized with TEA, TEC, DBP and DOP exhibited similar tensile strength in the range of 20 – 22 MPa, however high values of elongation at break (24 – 26%) showed only samples containing TBC, DBP and TEA. In case of CAB hybrids the introduction of inorganic phase into polymer matrix caused hardening and reinforcing of the material, thus an increase of tensile strength in comparison with unmodified CAB was observed. Regarding organic-inorganic hybrids prepared from 93.75/6.25 and 87.5/12.5 polymer/TEOS formulations the highest values of tensile strength (23 – 24 MPa and 25 – 27 MPa) were obtained for samples 6.25TEA25, 6.25DOP25, and 12.5TEA25, 12.5DOP25, respectively. However, at the same time, obtained samples exhibited lower values of elongation at break as compared with plasticized CAB, due to the higher brittleness of the material. The results showed that the presence of 25% of plasticizer in organic-inorganic

Considering the effect of plasticizer concentration it can be concluded that all of the plasticizers investigated, excluding TEC, caused an antiplasticization at concentration 30% of the plasticizer, resulting in an increase in tensile strength in comparison with the values at 25%. To the contrary, samples plasticized with TEC showed a common trend: with increasing plasticizer content, the tensile strength decreased, while elongation at break increased. Antiplasticizing effects were previously observed by Donempudi et al. for PVC membranes plasticized with phthalates [37], reported for citrate esters used as plasticizers for poly(methyl methacrylate) (PMMA) [38], and also has been found for polycarbonate, polysulfone, polystyrene plasticized with various plasticizers [39]. Even though the phenomenon of antiplasticization has been already long observed in synthetic polymers, the mechanisms involved are not perfectly known. According to Anderson et al. the phenomenon can be attributed to a chain end effect. Antiplasticizers initially fill unoccupied lower volume at the chain end and then the overall polymer free volume. Chain end mobility is restricted, resulting, thus, in higher modulus and resistance, generally followed by polymer hardness. Jackson and Caldwell suggested that antiplasticization can be attributed to a free volume reduction due to antiplasticizers [40]. Another explanation is an increase in the degree of order or the crystallinity of the system, resulting in an increase in tensile strength. Antiplasticization of the samples may be attributed to the hindered local mobility of the macromolecules, and thus reduced flexibility, due to the strong interaction between polymer and plasticizer (i.e. hydrogen bonding, van der Waals' forces) [39, 41]. Antiplasticization in polymers depends on molecular weight and concentration of the diluent and occurs over a concentration range below the plasticization threshold. This point, dividing antiplasticization and plasticization behavior, is typical for each polymer– plasticizer system [42]. Gutierrez-Villarreal [38] reported an antiplasticization effect for PMMA plasticized with TEC at low concentration of plasticizer (about 13 wt%). The plasticization threshold for TEC plasticized samples based on CAB was not observed in the range of concentrations used in this study. For the samples prepared with lower concentration of TEC (below 25%) the measurement using a universal tensile machine was
