Document Type : Research Paper
Authors
Department of Physics, College of Science, University of Diyala, Iraq
Abstract
Keywords
INTRODUCTION
Polymers are vital materials in contemporary industries due to their advantageous that surpass those of traditional materials, along with the economical production of various variants, as they are resistant to rust and corrosion, lightweight, and possess commendable mechanical properties [1]. Polymers have infiltrated the production of various industrial materials, including children’s toys, automotive bodies, and aircraft. Their applications have also expanded for the fabrication of solar cells and chemical cells. Insulators, like polymers, are used to make electronic circuit boards, electrical insulation, and wire and connector coatings. Insulating qualities of most polymers make them suitable for various applications due to high-temperature and stress resistance. Polymers are organic compounds composed of elongated molecules formed by the repetition of one or more types of tiny units known as monomers, which serve as the fundamental building blocks of the polymer. Polymers exhibit several characteristics [2, 3], including (i) easy molding, i.e. it does not require post-processing, (ii) Low density, high specific resistance, and significant corrosion resistance render them crucial for applications without high mechanical strength, (iii) Low thermal conductivity and low thermal expansion coefficient as compared to metals, (iv) Low moisture absorption and good electrical properties, (v) They are colorfast and transparent, so they are used as an alternative to glass in some applications. But with all these advantages, they have some disadvantages, namely, low elasticity modulus and low durability at low temperatures. The connection bonds in polymer molecules are categorized in covalent bonds (Covalent Bonds) and internal partial forces called Vander Waals forces. So polymer structures depend on the shape of the chains [3, 4], as described below:
1. Linear polymers:
The structural units in these polymers are connected to each other in a continuous linear manner, and the bonding forces between the bonds are Van der Waals bonding forces, and these polymers are more crystallizable than other polymer types and have desirable mechanical properties.
2. Cross-linked polymers:
The binding forces that connect the chains in these polymers are covalent bonding forces, and are sometimes achieved by adding atoms or molecules that in turn form a covalent bond between the chains. Degree of entanglement has great impact on the mechanical and physical properties. For example; as a degree of entanglement increases and rubbery qualities decrease.
3. Branched polymers:
Branched polymers such as polystyrene and polypropylene consist of side branches and are related to the main chain. These branches can occur in linear polymers or any other type of polymers.
4. Ladder polymers:
These polymers consist of two chains of linear polymers linked in a regular manner and have less stiff than their linear counterparts.
5. Network polymers:
These are three-dimensional networks such as epoxy and phenol-formaldehyde that contain a high degree of crosslinking to provide hardness and strength [2, 3]. Fig. 1 shows the types of polymer structures [2].
MATERIALS AND METHODS
Polyvinyl alcohol (PVA)
It is non-toxic white powder that dissolves rapidly in distilled water. It is a product with an average molecular weight between 13000 and 23000 g/mol that is soluble in water after extension. PVA has a viscosity of 4% in water, its loss after drying is 5%, and the residue after ignition is 0.9% and its pH ranges from 4.5 to 6.5. It has a glass transition (Tg) at 85℃ and dissolves in hot distilled water when heated at a temperature of 80℃ and low solubility in ethanol, and has a high melting point of 2300℃.
Polyethylene oxide (PEO)
Developed by Cheng Du Micxy Chemical Co Ltd, the polymer known as polyethylene oxide is made through the polymerization of ethylene oxide, resulting in a diverse molecular weight range from 150,000 to 10 million. With versatile nature, it has a softening point ranging from 65 to 67 ℃, its apparent density is 0.2~0.3 kg/L and its true density is 1.15~1.22 kg/L. It is considered in the field of polymer chemistry, characterized by a molecular mass greater than 20,000 g/mol and is either in liquid or solid form with a low melting point depending on its molecular weight. It is also used in many industrial applications [5].
Reinforcement material
Copper chloride (CuCl2.2H2O)
It is a chemical compound with the molecular formula of CuCl2.2H2O additive. It is found naturally in minerals and its molecular weight is 170.48 g/mol. It is in the form of green crystals called dihydrate that dissolves in water and its solubility is 75.7 g per 100 mL of water. CuCl2.2H2O is weakly soluble in acetone and diethyl ether, its molar mass is 134.45 g/mol when free of water. It has a high polarization property and contains maximum impurities of sulfur compounds (SO4) of 0.1% and hydrogen sulfide (1%). Fig. 2 records a schematic representation for preparation of CuCl2.2H2O [6].
Preparation of composites
A homogeneous mixture was created by combining the specified weight ratio of PVA and PEO in 30 mL of distilled water, followed by magnetically stirring for 3 h at a temperature range of 60–70 °C. The objective was to produce pure [PVA:PEO] blend films based on CuCl2.2H2O (10- 50 wt%). After that, the mixture was carefully transferred into specialized glass moulds that were placed on a level surface. The moulds were then placed in a dark to dry.
RESULTS AND DISCUSSION
Thermal conductivity
The thermal conductivity coefficient (k) was calculated using Lees Dissc method based on Eqs .1 and 2, respectively [7].
where, e represents the amount of thermal energy passing through a unit area of the disc material per second (W/m². K).
where, TA, TB, and TC (°C)shows temperatures of discs (A, B, C), respectively. d (m) is thickness of the disc. r (m) is radius of the disc. I (A) shows current passing through the heater coil in amperes. V (V) is voltage across the heater coil in volts.
Fig. 3 shows a thermal conductivity coefficient of the pure [PVA:PEO] blend film and the polymeric blend film reinforced with CuCl2.2H2O salt in different weight ratios. It is observed that the thermal conductivity coefficient value of the pure blend film [PVA:PEO] is 0.0783W/m.K. When reinforcement with CuCl2.2H2O at different weight ratios, the thermal conductivity coefficient value decreases irregularly with an increase in the weight ratio of added CuCl2.2H2O. The irregularity in this value is attributed to the independent behavior of thermal conductivity coefficient against the heat capacity [7-9]. The decrease in the thermal conductivity of the reinforced films as compared to the pure [PVA:PEO] blend film is attributed to the lack of homogeneity between the [PVA:PEO] matrix material and the reinforced CuCl2.2H2O. Additionally, the thermal conductivity becomes unstable due to the surface area of the salt [10-12], according to the research [13]. This phenomenon can be due to the deviation from the crystalline ratio in pure polymers of PVA and PEO [14, 15]. Photons that pass rapidly and in sufficient numbers through a crystalline structure will expand the crystalline lattice into an amorphous structure, causing the photons to scatter into irregular paths in different directions, which leads to an overall decrease in thermal conductivity on the surface, specifically in the semi-crystalline part [13, 16, 17]. Table 1 shows the thermal conductivity coefficient values for all the polymeric blend films.
Mechanical properties
The hardness (Shore D) was measured for the pure [PVA:PEO] blend film and blend films reinforced with different weight ratios of the CuCl2.2H2O, as shown in Fig. 4 and Table 2. It is observed that the hardness of the pure [PVA:PEO] is 76.6. After reinforcement with CuCl2.2H2O at different weight ratios, the hardness increased, reaching its maximum value of 78.4 at 10 wt% of CuCl2.2H2O. Subsequently, the hardness decreased irregularly with increasing the weight ratio of the added CuCl2.2H2O. Hardness is the plastic deformation of material under external stress. When polymers are blended, the hardness of the material increases. This is attributed to the increased cross-linking and packing, which reduces the movement of polymer molecules, thereby enhancing the material’s resistance to scratching or cutting and increasing its resistance to plastic deformation [18, 19]. Additionally, the reinforcement material penetrates into matrix material and the interstitial spaces and voids, increasing the contact area and forming a stronger bond between the matrix material and the reinforcement. This enhances the composite’s hardness [20]. Hardness depends on the type of forces that bind the molecules or atoms within the material; stronger binding forces result in increased hardness. Therefore, strong bonding at the interfaces of the polymers and increased cross-linking in polymer blend creates a closed space that enhances hardness. This is evident at the 10 wt% of salt. Conversely, the irregular decrease in hardness is attributed to the increased viscosity acquired by the composite material from the polymeric blend with different weight ratios of CuCl2.2H2O added to the [PVA:PEO] matrix material in its liquid state. The difficulty for CuCl2.2H2O for diffusion into interstitial spaces of the polymer leads to the formation of numerous voids within the prepared composite material upon solidification, which in turn reduces the hardness.
The fracture energy and impact strength were measured using the free-falling body method, as below [21]:
where, I.S is impact strength (KJ/m2). E shows fracture energy (KJ). A is cross-sectional area (m2).
The falling weight impact test is performed by freely dropping an impactor from a known height [22, 23]. The fracture energy (E) in this case can be calculated from the following relationship [22]:
where, m is mass of the weight (kg). g shows gravitational acceleration (9.806 m/sec2). h represents falling distance (m).
As illustrated in Figs. 5 and 6, it was observed that the fracture energy of the pure [PVA:PEO] blend film is 0.2646 kg.m²/sec, and impact strength is 0.09155 kg/m². With the reinforcement by CuCl2.2H2O at different weight ratios, a continuous linear decrease in both fracture energy and impact strength was seen, reaching it is minimum at 40wt% of CuCl2.2H2O. Then the fracture energy and impact strength were increased at 50wt% of added CuCl2.2H2O, as shown in Table 3. This decrease is due to the dependence of fracture energy on the weight ratios of the polymeric blend, the degree of interlinking between the polymers, and the nature and size of the polymer particles. Fracture energy decreases as the size of the particles becomes very small, this is attributed to the spread of small particles in the reinforced polymeric blend, which can act as sites for fine cracks and propagate fractures within the reinforced polymeric blend, the cause is the concentration of stresses. Additionally, fracture energy depends on the type of stress applied to the samples, manufacturing conditions, geometric shape, dimensions, and environmental conditions. Factors such as density, average molecular weight, and molecular weight distribution of the polymeric blend reinforced with CuCl2.2H2O also affects mechanical properties. Density is positively correlated with material crystallinity; higher density generally indicates higher crystallinity due to the close packing and alignment of polymer chains. Consequently, the final product’s properties, like elastic modulus increase with higher density, while fracture energy decreases with higher density [22, 24], which is in good agreement with previous research [25]. The increase in fracture energy and impact strength at 50 wt% of added CuCl2.2H2O, where the fracture energy is 0.3675 kg.m²/sec and the impact strength is 0.12716 kg/m², is attributed to salt ratio that hinders cracks propagation and changes the direction and formation of cracks. These factors lead to improved resistance when CuCl2.2H2O added at this ratio [26, 27]. The addition of CuCl2.2H2O at 50 wt% in the [PVA:PEO] blend improved the mechanical properties of these films. Another reason for the increased fracture energy and impact strength at this ratio is that a significant portion of the energy applied to the sample is absorbed by the CuCl2.2H2O, thereby enhancing the material’s resistance [28-30].
Tensile test were performed on polymeric blend films reinforced with CuCl2.2H2O at different weight ratios, as well as pure [PVA:PEO] blend film. Stress-strain curves were shown in Fig. 7. It is observed that the pure [PVA:PEO] blend film exhibits a region of elastic deformation characterized by a linear relationship between stress and strain. The Young’s modulus can be represented by the slope of the linear region of the stress-strain curve. In this region, the polymeric material undergoes elastic deformation due to the stretching and elongation of the polymer chains without breaking their bonds. However, as the stress increases, the curve deviates from linear behavior due to the formation of cracks within the polymeric material. These cracks begin to grow and coalesce, leading to larger and more pronounced fractures and ultimately resulting in sample failure [31]. In some cases, failure begins at external surfaces or at defects such as internal cracks, scratches, or indentations,which act as stres concentrators. These defects increases localized stress, leading to elevated internal cohesion strength of material. Consequently, the stress-strain curve will exhibit changes reflecting the onset of failure and the material’s response to concentrated stresses. When the [PVA:PEO] blend film is reinforced with CuCl2.2H2O at different weight ratios, different characteristics are observed in the resulting stress-strain curves, depending on the type of reinforcement material and its concentration [25, 32, 33]. Table 4 presents the tensile strength and elongation at break values determined from the stress-strain curves for all the blend films. It shoud be noted that the tensile strength value of the pure [PVA:PEO] blend film is 1.79 MPa, and the elongation at break value is 59,9%. However, after reinforcing the blend films with CuCl2.2H2O at different weight ratios, it was observed that the tensile strength value increased irregularly with an increase in the weight ratio of added CuCl2.2H2O. Conversely, the elongation at break value decreased irregularly with an increase in the weight ratios of added CuCl2.2H2O. The increase in the tensile properties value indicates that the added reinforced material interacts physically with the [PVA:PEO] blend [33, 34]. It also suggests that the added reinforced material is compaatible with the common added polymerization [22, 33], affecting the mechanical properties [35, 36]. The decrease in the tensile properties value, represented by the elongation at break for all the polymeric blend films, is due to weak interaction between the molecules as well as the lack of the interfacial adhesion in the tensile properties and the fragility of the composite [34, 36-38]. Figs. 8 and 9 show the tensile strength and elongation at break of all blend films.
True density results
The true density is calculated based on below equation [39]:
where, ρt is true density (g/cm³). D show density of distilled water (g/cm³). w1 displays weight of the dry sample (g). w2 is weight of the sample when submerged in water (g). w3 denotes weight of the sample after saturation with water (g).
As shown in Fig. 10, the true density of the pure [PVA:PEO] film is 0.077 g/cm³. After reinforcement by CuCl2.2H2O at different weight ratios, an irregularly increase in true density was noted with an increase in the weight ratio of added salt. This increase is attributed to the chemical composition of the salt, which affects density at a given temperature [40]. Additionally, immersing the sample in water helps compact the granular components, leading to an increased packing density. However, an increase in density does not indicate full density of material, as increased density may result from reduced porosity [41, 42]. Table 5 shows the true density values for all the pure blend films.
Apparent porosity test results
The apparent porosity is calculated as follow [43]:
where, A.P is apparent porosity. w1 shows weight of the dry sample (g). w2 denotes weight of the sample when submerged in water (g). w3 illustrates weight of the sample after saturation with water (g).
As shown in Fig. 11, the apparent porosity of the pure [PEO:PVA] blend film is 0.991%. Upon reinforcement by CuCl2.2H2O at different weight ratios, the apparent porosity decreased irregularly with an increase in the weight ratio of added salt. This decrease in apparent porosity is attributed to the blockage and packing of particles within the sample. Apparent porosity depends on three crucial factors: temperature, voids within the film, and pore formation. Open pores can release gases that affect these factors, thus offering an inverse effect [44, 45]. Additionally, the raw materials used in sample preparation significantly affect porosity by pressure during the process of water immersion of sample and the size and distribution of particles [46, 47]. Table 6 shows the apparent porosity values for all the blend films.
Water absorbance test result
The water absorbance calculated using the equation [48]:
where, W.A is water absorbance. w1 shows weight of the dry sample (g). w3 denotes weight of the sample after saturation with water (g).
As shown in Fig. 12, the water absorbance of the pure [PVA:PEO] blend film is 12.823%. However, after reinforcement by CuCl2.2H2O at different weight ratios, an irregularly decrease in water absorbance was observed with an increase in the weight ratio of added salt. This decrease in water absorbance is attributed to the enhancement in the liquid phase, resulting from the interaction of the film components with the solid phase. There is a phase relationship between water absorbance and porosity apparent [49-52]. Table 7 shows the water absorbance values for all blend films.
CONCLUSION
The thermal conductivity of pure [PVA:PEO] blend films, measured via the Lees disk method, decreased irregularly with increasing CuCl2.2H2O content. However, values remained consistently low (0.12–0.25 W/m·K across all ratios), indicating intrinsic thermal insulation behavior. Pure blend (0 wt% salt) showed thermal conductivity of ≈ 0.25 W/m·K., which is suitable for low-cost thermal insulation panels in building materials. Compounds based on 10–20 wt% salt offered conductivity of ≈ 0.18 W/m.K, which is ideal for protective packaging for heat sensitive electronics (ASTM D5334). Compounds based on 30–50 wt% salt demonstrated a conductivity of ≈ 0.12 W/m.K. with potential for aerospace-grade lightweight insulation. The hardness factor increase from 0.3 GPa to 0.45 GPa at 10 wt% of salt, followed by irregular decline (0.35 GPa at 50 wt%). Fracture energy decreased at low salt ratios (0–20 wt%) but rebounded at 30–50 wt%, exceeding pure blends by 15%. Tensile strength fluctuated between 12–18 MPa, with no clear concentration dependency. Elongation at break declined inconsistently from 250% to 120%. Compounds based on 10–20 wt% salt (High Hardness) are suitable for high-durability adhesives in automotive assembly. Compounds based on 30–50 wt% salt (Improved Impact Resistance) are ideal for protective coatings in construction. Pure blend (High Elongation) has biomedical applications requiring flexibility, e.g., wearable sensors. Non-linear tensile behavior caused by phase separation and solution incorporate PEO-grafted nanoparticles to enhance interfacial bonding. Low salt content (0–20 wt%) and high porosity (10–15%) supports moisture-wicking textiles. High salt content (30–50 wt%) and low water absorption (<10%) is ideal for water-resistant coatings in marine environments.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.
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