Dimensional accuracy of an impression material is a very important aspect when considering the replication process of the oral soft and hard tissues . An impression accuracy depends on many factors, such as chemical composition, setting reactions, by-products, and disinfection [21, 30]. Although an exact dimensional replica may be very difficult as many of the steps result in dimensional changes, some of these changes might compensate at a certain point for one another [31, 32].
A metallic mold was used in the present study for evaluation of the accuracy to ensure dimensional stability of the master model during the whole testing period and under different environmental conditions. Moreover, type VI stone was used for preparation of the stone casts because it is highly compatible with elastomers and is considered very accurate in recording the fine details as well. In addition, to overcome the great deal of errors that usually result from the analogue method, a high-resolution digital camera with an AutoCAD software was used to measure the dimensional changes.
The results of this study showed that all the tested materials coincided with the ADA specification no. 19 that recommends a maximum negative change in dimensions to be 0.5% after a minimum of 24 h, and the ISO 4823 specification which allows a dimensional change of less than 1.5%. This was in agreement with previous reports stated by Mandioks M. et al. , Chai J. et al. , Faria A. et al. , Stober T. and Schmitter M  and Katyayan PA. et al. .
In the current study, the polyvinylsiloxane impression showed the highest dimensionally accuracy among all tested materials followed by VPES, whereas the PE showed the least accuracy. The high accuracy of PVS could be attributed to the excellent physical and mechanical properties of this type of material including high elastic recovery, and good tear strength . Moreover, its setting occurs through an addition polymerization reaction between the silane and vinyl groups terminal groups with the hybrid group without the formation of by-product and material shrinkage .
Conversely, the newly formulated platinum-initiated vinylpolyether silicone (VPES) showed significantly lower dimensional changes compared to polyether that might be referred to its chemical composition. VPES consists of a copolymer of α-divinylpolydimethylsiloxane and α-divinyl polyether cross-linked by organohydrogenpolysiloxane . This composition intended to incorporate the natural hydrophilicity of conventional polyether impression materials along with the desirable properties of vinyl polysiloxane materials . For further wetting and flowability enhancement, the manufacturer has incorporated a surface tension eraser (STES) and wetting conditioner surfactant (WCS) into the vinyl siloxanether .
However, the high dimensional changes recorded by the PE group could be attributed to the ring opening cationic addition polymerization reaction. Each stage of the reaction and the ionized form of sulfonic ether acid involve the opening of an epimine ring. Each molecule of prepolymer has two reactive groups of epimine; therefore, induced propagation produces chain elongation, causing expansion .
According to O’Brien , wetting describes the relative affinity of a liquid for a solid, which can be quantified by measuring the contact angle. The result of the present study showed that the mean contact angle of VPES hybrids was (69.6); polyether was (79.2) and the mean contact angle produced by the hydrophilic polyvinylsiloxane (Express) was (87), one minute after mixing. This was in agreement with several studies [24, 40], which stated that polyether behaves more hydrophilic during the process of setting and can therefore exhibit better flow properties compared to PVS materials.
On monitoring the droplet every 10 s, polyvinylsiloxane had fast kinetics towards more hydrophilic equilibrium surface state compared to polyether. This possibly balances the disadvantage of the initial PVS impression materials hydrophobicity. Results of present study was similar to the conclusions drawn by Ruff et al.  and Kugel G et al.  who stated that, although surfactants were added to these materials, their hydrophilicity remained less than conventional polyether.
Polyether showed pronounced and constant initial hydrophilicity throughout the prescribed working time, which is consistent with Michalakis X et al. . This could be due to the polar polyether molecule that provides the material with an inherent hydrophilicity and low contact angle. Polyether impression material is more hydrophilic because of its functional groups [carbonyl (C=O) and ether (C–O–C)]. These polarized groups can attract and interact with water molecules .
Conventional polyvinylsiloxane behaves hydrophobically because it does not contain any polarized groups. Nonionic surfactants have been incorporated into PVS to overcome the inherent hydrophobicity . These surfactants act through a diffusion transfer of ionic molecules from the surface of PVS into the aqueous phase in order to reduce the surface tension of the liquid . As for the surfactant once it reaches the surface, lee DY et al.  believe it remains attached to the impression surface, while others postulated that the surfactant is released into the liquid at the interface .
VPES hybrids wettability results from its chemical structure as they have polyether functional groups and probably also to surfactants added to the material. The chemical structure of the hybrid material as claimed by manufacturers is formed of large polyether molecules as a backbone to which smaller polyvinylsiloxane molecules are attached .
VPES results in the present study reinforce Menees et al. findings . However, these results are not consistent with another study which claimed that only soft PE material is intrinsically hydrophilic (initial contact angle < 90°), whereas VPES is intrinsically hydrophobic (contact angle > 90°) .
Additionally, it was found that the time of the droplet placement in relation to the working time affects the wettability, where for additional silicone higher contact angles were observed at the end of the working time. This can also be explained on the same concept of surfactant migration as it may be difficult for the surfactant to migrate while polymerization and cross linking proceeded at the end of the working time .
On the other hand, polyether and VPES hybrid reported higher contact angle at the end of the working time compared to that obtained one minute after mixing. This could be attributed to the mobile polymer chains at the start of the working time, however at the end of the working time, the polymerization reaction and the polymer network formation would have progressed enough to cause more steric hindrance that decrease the interaction with water molecules [50,51,52].
With regard to detail reproduction, all the three tested impression materials in this study reproduced the three lines under dry conditions. Therefore, the three impression materials complied with the ADA requirements for detail reproduction of elastomeric impression materials. These results are in agreement with Petrie CS. et al. , who concluded that the best surface details were obtained only under dry conditions for elastomeric impression material.
Under wet condition, there was a significant difference among the three tested impression materials regarding definition, sharpness and continuity of the 25 μm line. VPES hybrid and PE showed higher prevalence of well-defined reproduction of details. However, PVS showed higher prevalence of continuous details with loss of sharpness. Additionally, PVS was the only material that showed loss of continuity.
These results were in agreement with German et al. , who concluded that the hydrophilic polyether material is capable of reproducing fine details on moist surfaces while, many polyvinylsiloxane products are not capable of yielding the same results. This could be due to hydrophilicity, which is the main property that affects wetting of oral soft and hard tissues and correspondingly, affects the accurate detailed reproduction of prepared tooth surfaces [54, 55].
The inherent hydrophilicity of polyether accounted for its superior behaviour in moist conditions, however for vinylpolyether silicone hybrids the presence of both polyether and hydrophilic addition silicone in its chemical composition are responsible for the reasonable detail reproduction supported by the adequate hydrophilicity.
Results of this investigation reinforce postulations stating that the so-called hydrophilic polyvinylsiloxane material remains hydrophobic in the un-polymerized, liquid state and will not adequately wet moisturized surfaces [40, 46]. Though incorporation of hydrophilic surfactants, which commonly consist of an oligoether or polyether substructure, improves the distinct hydrophobicity of conventional PVS, it appears that the impression material still cannot accurately reproduce the details in the presence of moisture [56, 57].